Every part of loot you get takes up varying levels of room in poe orbs your luggage compartment. To take as many possible things at once, you have to move things around and squeeze items into the allotted spaces. Even then, your luggage is likely to fill up quickly if you pick up every single item you encounter.
The trick, then, is to ignore items that your character can not use or are of reduced value. Item rarity is indicated by colors: White: Norma,Blue: Magic,Yellow: Rare, Orange: Unique.
You can pretty much discount white things unless you're going to go back to Buy poe chaos orbs town and have the room to spare for them. Rarer items are often worth picking up and selling, even if you need to hold onto them for a while. And needless to say, any item that appears better than your existing gear is well worth catching -- even if you have to lose something else temporarily while you equip it.
Don't neglect daily life and mana regeneration
By default, figures do not regenerate life but they do poe equipment regenerate a tiny amount of mana every second. You can manually synchronize life and mana by using the appropriate kind of flask. Flask charges automatically refill when seeing a town or hideout. In addition they gain charges from killing enemies. Equipping better flasks increases the quantity of life or mana regained, and can have other effects as well.