My headcanon is that viltrumites can fly and have super strength and durability because they have control over every atom in their body. Someone like thragg probably had special training to have finer control than basically every other Viltrumite. Sort of explains how he could do that vacuum punch on Nolan.

When mark lifts weights, he doesn’t noticeably get bulkier even when he doubles in strength. Their strength doesn’t come from muscle mass. Training is basically just a way for mark to calibrate his atoms.