Version: Unreal World 3.63, Linux/Steam build, unmodded
What happens: I have a tame reindeer doe that can happily lift 150lbs in slender trunks, but I get "Burden of X is too much for Reiny to carry!" messages after packing on variety of small light items, such as feathers or dried meats.
What's expected: Reindeer and other animals can carry the same weight, regardless of what that's composed of.
Notes: It appears the reindeer can only carry 20 inventory entries, and once they're filled it can't take any more. This makes it poorly suited for keeping my inventory clear of miscellany, such as feathers, small furs, food, cords, and tools. ;(
I've made a snapshot of my save which I can provide upon request.
Edit: Many folks have mentioned the item limit is by design, but that design decision is not exposed to the player, as there's no difference in message between exceeding the item limit or the weight limit.
Having separate messages for each ("That is too heavy for X to carry" vs "You can't find a way to organise that many items on X"), or having the animal inventory display how many slots remained, would make it clear which limit is being reached.