The way I understand it, is on ave, mule deer are bigger, as in thicker, like comparing a neanderthal to a ***** sapien. Whitetail are generally smaller than mule deer but can vary a lot more in body size. WNY has a good population of large bodied whitetail, over 200 lbs dressed, which can be bigger than some mule deer.
Also, did you know that as a species, whitetail deer are around 16 million years old iirc. It's funny, but humans are one of the newest kids on the block.