1B+ ruminants with similar size / digestive systems? (including buffalo, aurochs, rhinos, etc) Absolutely. There were 30-60 million bison roaming the Great Plains just 170 years ago (not far off from America's current ~90 million head) [1]. The size of ruminants in the ice age and prior was also much bigger than modern cattle [2] [3].
[1] https://allaboutbison.com/wp-content/uploads/2019/05/Bison-P... [2] https://softschools.com/facts/extinct_animals/aurochs_facts/... [3] https://www.theextinctions.com/articles-1/ns0atubs8qd60o16it...