In the series, Giant-Man has normal human strength until he grows. Upon growing, however, he can lift something as heavy as a ship (and chuck it at Graviton). To me, that means that Giant-Man has a Brawn 1 that gets augmented once he activates his size increase. The problem is that while Growing in BASH UE grants a +5 Result Modifier per level to Brawn related checks, lifting isn’t handled through such checks. By the rules, then, Growing doesn’t actually affect what a character can lift.
A quick point- Growing DOES affect what you can lift.
Your weight is also greatly increased by
growing, but you can lift one half your weight (or the amount indicated by Brawn if it is higher).
So if you have Growing 5, you can lift a MINIMUM of 75 Tons. That's like a Brawn of 4, bordering on 5.
By the normal rules, you don't usually roll to lift something though- the benchmarks tell you what you can lift. If you want to specify, though, you can require rolls for someone trying to push their limits.