You're making the bold mistake of thinking that he's transforming into Super Saiyan from his Ultimate form, he's not. He's in his Base form and he's trying to acquire Super Saiyan, but his body is far too rusty to be able to achieve this.
Teen Gohan
a) Larger muscles, much larger build.
b) Can willingly transform into a Super Saiyan without an issue. Heck, he can even willingly transform into a Super Saiyan 2.
Adult Gohan
a) Much smaller muscles, much slimmer build.
b) Has an extremely difficult time maintaining Super Saiyan and powering up.
Lookit the difference: