Everything has a maximum item (stack) limit. Animals, rafts, companions,... The PCs item stack limit is higher than the other limits, but there's a limit for the PC as well. Everyone can carry a stack of 10000 black grouse feathers, as that is a single item stack. Carrying capacity is thus limited both by weight and stack count, whichever is hit first. Thus, have your bull carry your heaviest stacks and the PC the lighter ones.