Questions regarding how Superman's powers work and what role Krypton's gravity has on him rumble on eternally, writers still like to remind us that Krypton's high gravity plays a part in what he can do but never actually explain what it is that the gravity differential does for him...
But check out one of the earliest Superman Origins and actually, it all makes a neat sort of sense:
Now that's how it should be.
That explanation still most sense to me, as it was always stated/implied both factors were in play, and that he should be always somewhat super, regardless if Sun or not!
And could state that he can mentally control his body desenty factor, so that he weighs normal weight on earth!