Pretty sure everyone besides Goten & Kid Trunks on Z got buffer (or more defined) when they transformed into one of those forms. Arguably even the kids, to a smaller degree as well. It's not been a thing on Super, but it most definitely was a thing during Z.
The only Saiyans that bulked up after turning Super Saiyan was Goku going Super Saiyan and Super Saiyan 3. Everyone kept the same muscle mass, unless you count Ultra Super Saiyan and Super Vegeta. In fact, a major point of Super Saiyan 2 was how it was sleek.