Not quite. Donkey Kong used to be a large and intimidating character, much bigger than Mario. Donkey Kong Jr. was always shown to be ever so slightly bigger than Mario. (Shigeru Miyamoto once said in an interview that they wanted the player to control Donkey Kong in the sequel, but because the character was too big for the screen they introduced a "little Donkey Kong" instead - Donkey Kong Jr.)
Junior's design was slightly inconsistent in the early days, but generally he was a chunky kind of fellow with broad shoulders. In Game Boy Donkey Kong his design was tweaked a little - he became slightly smaller than Mario, his features were rounder, his shoulders were not so broad, he had two little hairs on the top of his head and the J on his shirt once again became a solid red with no outlines. This design hasn't been strayed from since (Mario's Tennis on Virtual Boy aside).
In Donkey Kong Country, although Mario wasn't present for a true comparison, the impression was that Donkey Kong had become a lot smaller (yeah I know, different generation, blah blah) - now only slightly larger than a kid/teenage monkey, and towered over by his nemesis K. Rool.
Once Donkey Kong rejoined the Mario cast with Mario Kart 64 the size change was immediately apparent - he was only barely bigger than Junior was in the original game!
A similar height reduction was felt in the Game & Watch Gallery series - Junior was about the right size in all of the games, but Papa Donkey Kong was again only a little bit bigger than Mario. Though this is probably down to the fact that the games required a lot of action to fit on one non-scrolling screen, and DK was quite small in most of the original LCD G&Ws anyway.
This isn't taking mushroom effects into account though mind. As Game Boy Donkey Kong's ending showed us, we were only playing as the regular Mario, not the mushroom powered Super Mario!
