A common rule across multiple systems is some sort of Encumberance rule. Encumberance dictates how much a character can carry before they're overloaded. Some systems do it on weight. Some systems do it based on the size of the object. Others do a combination. Thing is, I've yet to see an encumberance system that didn't have people balking about it and the limitations that come - or don't come - with it.
Today, I want to talk about Encumberace, reality, and what the two may mean for your game.
It's More Than Just Strength
The first thing you need to know about encumberance - for the real world - is there is a lot more to it than just strength. This is why size of the objects comes into play in some systems. Simply put, a ton of marshmallows is going to be more encumbering than a ton of iron because iron because the marshmallows are going to take up a lot more space - while weighing a ton - and thus be harder to carry. Put another way, ever help a friend move some furniture? Often the complaint isn't the weight, but the size and awkward shape of the object that poses problems.
This is important to understand when considering encumberance and how you want to do it for your game. It's not just a matter of weight. Bulk, size, and distribution of the weight is also important.
Where You Carry It
Medieval armor could be anywhere from 40lbs to 70lbs of weight. That sounds like a lot, but it really wasn't as restrictive as most people think - and there are tons of videos on youtube of people tumbling in full plate to prove it. The reason for this is where the armor is worn and how. Most of the weight for armor could be put on the hips, letting the body's full strength carry it without limiting momentum. The armor above the hips was then also supported by the arm below as well as the user's shoulder. This distribution across the body made it feel lighter than it was - especially in motion. In the end it works out light a firefighter's gear, or a modern day soldier's full kit.
The same is true for the equipment that adventurers and PCs carry. The bodyis capable of holding and transporting a lot more than it looks like provided it is distributed evenly across the whole body. Look up pictures of soldiers' kits through the ages and you can see just how much stuff people carried. Look at what some folks have for their hiking gear, camping trips, or survivalist expeditions. It is impressive.
The catch though is that when you are carrying like this it is obvious that you are going around carrying a bunch of stuff. There's a pack on your back, pouches on your belt, chest, and thighs. You can't look sleek and cool while also carrying a ton of stuff. Though, you can still look cool. Just in that "I'm loaded out to go to war" sort of way.
Weapons On Your Back
Historically people carried weapons on their back for one primary reason: transport. When I say transport I mean the person needed their hands free - or wanted their hands free - while traveling. They did not wear weapons on their back to travel. The reason for this is simple: it's a pain in the ass - if not flat out impossible - to draw a weapon from the back, and even harder to put it back there once the fighting is done.
This is why you rarely, if ever, see people in movies draw swords from their back. You'll see them reach for the sword and start to draw, then it cuts away to show someone else - usually the opponent - and when you next see the character they have the weapon drawn.
The same is true with arrows but for different reasons. I have a friend who is quite good, and fast, at drawing arrows from his back. However, when passing under trees, or other overhead obstacles the arrows snag. Not to mention spilling more easily when you get tossed around from the necessity of a back draw.
I bring this up because carrying things on the back is a great way to have more storage room, but accessibility becomes a major concern. Especially on the spot access, like when being jumped for combat.
So what could an adventurer carry realistically? Some of this does depend on their strength. In general though, weapons the person wanted access to for on the spot fighting (random encounters) would want to be in hand, or worn on the hip. Two handed weapons (great swords, spears, pikes, etc) would need to be carried by hand. Things like knives could be worn on the chest.
More could be carried on the back, but these weapons would take longer to access and would likely need help or some time to be put back when not in use. Back draws are possible for some weapons, but back sheathing would need to be practiced.
And In Game?
Well, is your game realistic? D&D is definitely not realistic, nor does it care about realism as of 5th Ed. It is awkward to have a greatsword and a pike at the same time, but D&D does it without batting an eye. It also enables weapons to be drawn as part of the use action, and to be put back away without much concern. D&D 5th Ed also assumes that even at level 1 you're something of a badass with lots of training and experience though.
But that's Rules As Written, and you'll find most games don't particularly care if a character has a gun hidden in the crotch of their underwear in terms of getting it out and using it in combat. If you do? Maybe consider just where and what people can carry. It's a lot more than some simple number comparisons.