We saw that in Flashpoint that without the sun Kal is thin and sickly.
Yellow sun makes him "magically" strong, but it also inflates his body.
There was a Louise Simonson story form the 90s, where Superman got some sort of over dose of sunlight and he hulked out. It didn't look healthy and it wasn't healthy. It appeared as if he was about to pop.
According to the TV show, it was her magic girdle that gave her strength and if she took it off she was suddenly only human strengthed.
But then Diana is a golem given life by the breath of the gods, in "some" continuities, which is why Amazons in general in some continuities are not as strong as Superman, which is why the event Amazon's attack was a bit shitty. A couple thousand non powered women with swords and spears attack America and (almost) win? They weren't even bullet proof for fucks sake!