There are only 2 species of Bison left on the planet. The ancient, ancestral (extinct) ones are larger than the ones today (The Aurochs of Europe is one, and is part of ancestral memory).
The current European Bison is similar in size to the American Bison – European (bonasus) is longer, while the American bison has the bigger spinal hump across his shoulders.
As a spiritual animal, BIson speaks of abundance. He provides for the entire tribe, all that a human needs. The entire bison is used for food, hides, unguents, soaps, sinew, glues, containers, tipi. That the bison was a baby shows that this abundance is yet to come. A full adult Bison thunders into his choices with confidence, a baby one takes 2-3 years to reach maturity.