The Big Haul

One of those weird and often-overlooked logistical issues is how your characters haul things around. Some of this is on a large scale-- how to supplies or trade goods get from Point A to Point B? Some of this is on a smaller scale-- how does your character get their supply of ducks to market, or their massive supply of weapons to wherever they're headed?

For one thing, it's a good idea to think about how much the items actually weigh. Remember to think about things like armor or cold-weather clothing, which can be extremely heavy in their own right. This isn't just about the upper limits of what your character can physically lift and walk around with, although you should really think twice if your 'petite' Mary Sue character is happily skipping around with 2/3 of her own body weight in her travel pack (unless there's an in-universe explanation-- in which case, carry on). Hikers and backpackers generally say that a reasonable maximum load is 25% of one's body weight, although an athletic person (or someone hopped up on adrenaline) can carry a much larger weight for a short distance. There's also considerations whenever your character hops onto a load-bearing surface, such as a suspension bridge or an ice floe. Similarly, someone carrying an extremely heavy load may have trouble navigating squishy terrain like sand, snow, or mud, as they will sink in deeper and require more energy to take a step.

Once we've figured out how much weight your character can carry, we can then determine the physical limits of what they can bring along on their quest to save the world without enlisting the help of genetically modified llamas or magical bottomless backpacks or the like. This sets other limits for your characters, such as how many day's worth of food they have, and whether they can bring a backup katana. Again, remember that the 'stuff in the backpack' weight limit includes whatever the character is wearing, which can be a significant amount-- for example, if I put on traditional Highland dress and weapons (scaled to my height), I've already used up almost a quarter of my weight allowance. If I substituted chain mail or plate armour, I'd have used up almost 2/3 of my weight quota.

If you introduce a pack animal or the like, you've got a whole other set of concerns on your hands. Either way, I think imposing limits and challenges on your characters by using the mundane, practical considerations of your setting is a good way to keep the plot going (even if the character is trapped by the weight of their backpacks).