My way of doing things involving the -massive-, size-ignoring inventories of the characters is to either severely nerf it IC, or use some sort of device that shrinks things and stores them in the slots. But I'm wanting to know what -you- people use to explain it, or do about it, because it's been bothering me for a while.
Basically, carrying a reasonable amount of stuff on the character, allowing more if there's bag/pouch etc. and storing stuff at different places: ship, house... I've seen before playing with a shrink device, I find that cheesy. (Mostly used to spam whatever is needed right at that moment or lotsa weaponz. Not cool). (How many belts do you have? ALL OF THEM)
Typically, though, what I do is my hotbar and one or two lines of my inventory is what I have IC. other than that, the rest is typically considered OOC.
Overall, as has been stated, it's generally just a good plan to think of what realistically would be possible for your character to carry. Carrying something like... a sidearm on their hip, rifle on their back, backpack with assorted equipment inside, etc. That stuff is acceptable. Carrying something like three rocket launchers, five different types of heavy rifles, a gattling gun, and a giant shipping container.... is not okay.
*Fills inventory with shipping containers, then proceeding to take them out of my Ass-Tardis and stack up to the sky.*