I believe that the relevant tag for carrying capacity is [trADE_CAPACITY:x]; wagons have it at 15000. If you just add [trADE_CAPACITY:45000] to every pack animal, you should get a lot more stuff. The limit will still be there, but will be ludicrously high; I don't believe you can actually remove it altogether, since the pack animals don't have it by default. Actually I just looked and all of the pack animals do have it, at between 1500 (donkeys) and 3000 (camels), with all others at 2000. I'd guess that it defaults to 0 if not present, though I suppose not including it is worth a try. It might default to infinity, after all.
The relevant entries should be in creature_domestic, creature_large_tropical, and creature_large_tundra. Muskoxen are in tundra, camels in tropical, and the other pack animals in domestic. Also, wagons are in creature_equipment if you want to mod them to carry more as well.
...I seem to remember someone somewhere saying that there actually is a set amount that the traders try to bring and that carrying capacity is the sole limiter. That was supposedly why elves who are happy with you bring only cloth; they quene a lot of cloth, put bins on the animals first, and fill up the weight limit with cloth bins before adding anything else. Radically increasing carrying capacity should basically make traders bring everything they wanted to.