I read somewhere that those bags for which you turn in the leaves were supposed to be only for "collection items", but they're bugged so you can put anything in them. I'm not sure what's meannt by "collection items" because they don't take up bag room after you add them to your collection anyway. Maybe they hold tradeskill items. Since someone said the Antonica quest yielded a 12 slot, maybe they dropped the restriction and, as a compromise, the number of slots.
I just finished the EL leaves collection, but I haven't turned it in yet, so I'll try to remember to let you know what I get.
There's another regular bag quest in EL - inspect one of the bags lying around in the Mill at the top of the hill on the easy side of the wall. You need to collect 15 Briarpaw pelts (about 1 in 5 drop off Briarpaws, not greater, unseasoned or cubs, just plain Briarpaws) You get a 10 slot.
There's also the Bag of Sewn Evil Eye quest, but I haven't started that one so I know little about it.