This is a realistic approach for a hardcore gameplay, do not expect to run with a whole plate armor set until you invest to all weight perks and get you strength high.
An average breast plate piece weights 10lbls(Vanilla), and with this mod your lvl 5 strength carry limit is 45 lbls without perks.
Keep in mind that you can always fight encumbered if you need the extra protection and you expect a difficult encounter.

Tested and working with KCD version 1.2

My take on the subject:

Weight in KCD is in lbs(pounds) so I was always bothered by the fact that you character could carry over 300 lbls of stuff sprinting around and fighting. 

For reference, 1 pound(lb) is 0.4 kg, and an average camping backpack is at most like 20kg ( I used to hike a lot :) ).
So after some research I think that a well trained knight could carry at most 60kg of gear (armor and inventory)-that is 130lbls in game.

About the plate armor weight values they vary from era to era, so when deciding what is historically accurate I took some artistic liberties - everything ofc based on official and solid articles and very reliable buy your own plate armor set today sites(lol really I am that guy).
My thoughts were that there should be a way to limit just putting on full plate armor straight from the start and snowball everything, and rather progress and invest to be a full kitted knight.
In KCD 2 it seems the armor weights are set only for for a single piece of armor when considering gauntlets, arm plates and leg plates so as far as Henry has two legs and arms the weight of the spoken pieces should be double that. Two gloves, double the weight.

Plate armor now is heavier now but weight perks are much more rewarding. You can always wear almost a full plate armor set around 15ish strength with all the weight perks unlocked.

For the horse part a horse can comfortably carry on average 20% of its body weight.
Lets assume that an average horse weights 1000 lbs(almost 500Kg) then it can lift 200lbs(100kg).

What changed:

Realistic Weight:

• Base horse carry weight limit adjusted according to realistic values.
• Adjusted saddle weight bonus up to 60 or so for the most expensive saddles.
• Reduced Henry's base inventory weight limit down to 40 with the use of a new custom perk.
• Hard working lad, pack mule,  strong bull perks give +10 bonus weight each
• Bonus weight from each point of strength decreased from 8 to 1
• Well built perk gives +1 extra weight point per strength
• Adjusted all localization files so the descriptions match the changes
  

Realistic Armors:

• All armor pieces and clothing weights readjusted so they match historically accurate values
• All clothes pieces readjusted so they match historically accurate values
• For every specific armor type piece, the better the quality the lighter it is within its acceptable weight range. So the more expensive the armor, more defense, less weight - all within the realistic acceptable weight range of the armor piece. 
• Same for clothes but to a much smaller weight range margin.
  

So the weight limit curve should be from 45(20kg) when you start as a lvl 5 Henry up to 130(60kg) incrementally as you make your way to lvl 30 and stacking up perks.

FAQ:

HardcoWhat?

Its the art of turning a game into a realistic sadistic sim. Imagine you run in your pants and someone with a big pointy sword stabs you IRL.If its the torso or the head you die right? If its the hand or leg maybe you lie down and then you die after another hit to the torso right? So I want this in my games. Realism.

WTF are you talking about, this is a weight mod?

Well you can do everything I imagine as hardcore experience with what we have here on Nexus - no map no icons damage armor no huds etc, besides simulate realistic carry weight, so thats what I contribute here.
Not for everyone, I get it.
There are other mods that just cut weight limit in half or so.
Not my scope here.

Dude wtf I cannot wear my full armor set this is S***

Well you can, you mean that you cant run right?
Heh, fun fact did you know that a full plate armor set was weighting like more than 30kg?(60 lbs)
So you came to a "realistic" mod page to complain that you cant run with your girlfriend on your back, right? (max limit is 130 lbls/60kg)
Not cool

What about this and that? I have really cool ideas man

If its doable somebody already did it elsewhere on Nexus, I have a life you know and I m here for fun.
Besides, I m a dev IRL but I have no clue really how KCD engine works (I do other stuff) and honestly we dont have any mod tools yet.
Also to be honest, I dont want to spend 500 hours to learn how to mod KCD just to play it for like 100, would you?
So please no requests, I will only accept bugs or game breaking problems. Well and nice thank you comments. I can live with those too XD

WTF man I dont see the changes you promised and I paid for - YOU SUCK!

This is probably due a mod conflict.
An easy way to see if thats the case is just rename the RealisticWeight folder in your KCD2 mod folder to something like ZzRealisticWeight so this would loaded on top of every other mod.
Another way is to just remove every mod temporarily besides the Realistic Weight and see if it works as intended.
You know, cut-paste-launch game, easy stuff.
If you find any issue with only the RealisticWeight mod in you folder - I SUCK - sorry it happens from time to time, report it in the comments and I will do what I can to help or upload a fix so you can get your moneys worth. 

What I paid for? But I downloaded this for free is there a paid version?

Lol no man everything here is free - its an old modders joke, relax. All modders are sharing their files with you for free and they dont get paid.

So why do you do and upload mods?

Am wondering the same from time to time. I have over 20 mods on my pc atm but uploading all that is a full time job. So bear with me, do your part and try to help and be respectful. To everyone here on Nexus really not just me. Have fun XD

Install instructions:

• Just unpack the zip files to your KingdomComeDeliverance2\Mods (create mods folder if it doesnt exist)
  

Compatibility:

• Its PTF so should be compatible with any mod that doesnt change the aforementioned perks or BaseInventoryCapacity and StrengthToInventoryCapacity in rpg_param.xml
• Realistic Weight does not touch any item weights or anything else relevant to item weight, except saddles.
• Realistic armor changes all armors and clothes in game so it should not be compatible with anything that changes specific armor values like prices and defense stats.
• There is a minor incompatibility with Hardcorish mod - it causes a conflict (I am not sure how and why) and Strong As a Bull perk does not work as intended. There is an optional version here that intergrades that mod's changes into this one, so you should delete that if use this version here. Please make sure to endorse the original mod if you use my version:
  https://www.nexusmods.com/kingdomcomedeliverance2/mods/578?tab=description

Everything here is safe to add/remove mid game, although keep in mind that this is close to an overhaul so in a first glance you have to
change you playstyle and maybe your build.

Known Issues:

• It seems that horse inventory limit is shared for Henry and his horse. So the horse inventory limit should also be reduced - this is not intented I ll see if I can do something about it
• Minor incompatibility with Hardcorish, strong as a bull perk will give +20 instead of the intended +10, working on a fix
• Horse saddles have a weight limit multiplier so you gain a lot of horse inventory weight. Am working on a fix
• Adjust harnesses weight limits increase - I m working on it. EDIT: Actually they are fine within realistic limits
• None other that I know of, comment and if something is broken I will see if I can do something about it.
  

I dont really intend to maintain this mod or take on requests, do whatever you want with it.
Search for KCD pak tool here on Nexus unpack the mod and repack it with the values you like
If someone wants to take on or include it in a mod feel free to do so.