As koteko said, animals have a much lower item stack limit than the PC, so you can easily hit that limit. I typically load the heaviest stacks onto the cow first.
The weight limit should be a lot above 100 (an average reindeer is 95, I think, and I think a large dog has the same limit). I think an average cow can carry a log (at least some of them can, as I've used a cow to move logs. It might be that small ones are too small, but I've never had any of those).
Also note that it seems to take the same time to push one item regardless of the size, so I ended up wasting hours when just pushing a pile of feathers to the side... Thus, carry stacks of many items on you character and load stacks of the same weight but with fewer items onto your cow (or dog) of burden.