Try the link I posted, mangeorge. There’s a bunch of sources that says the Rhinoceros is the 2nd largest mammal. Also, I don’t think the average size thing works. 
Look at it this way, if you used that logic, then the Elephant would be smaller than either Hippo or the Rhino. The Elephant’s nearest relative is the Hyrax which is about the size of a house cat. 
-Clueless