its both. everyone in Krypton is accustomed to a weak and white sun (I think its a white dwarf, and they're normal when around their weak sun. that's why they all died when krypton was exploding. they couldn't just fly away, cause they didn't have the power.

but when anyone from krypton goes near a strong, yellow sun, their bodies get stronger cause they aren't used to all this extra energy. or something like that.