I recall an interview with Cavill with him specifically saying he wanted to bulk up 100% naturally because it was fitting for the purity of Superman. Always thought that was pretty cool.
But yeah, actors do it all the time. Stallone went from kinda fit in Rocky 1 and 2, to beyond shredded in 3. Like, his 6 pack had it's own 6 pack. At least he admitted it. But I think I'd prefer Corenswet goes the natural route myself. Plenty of time to buff up naturally. Don't think I'd be appalled but not my pref.