Stoutstien
lunk
Yes and no.You sort of need to do both. Otherwise you end up like the 200 feet dinosaurs from Wotcs Glory of the Giants dealing 30 damage on a hit or something like that.
The issue with scaling damage with size is the fact that d&d has never handled size differentials well. You get to the point where there's not a difference between fighting a goblin at level one and a dragon at level 20 because you're practically doing the exact same thing which is a race to zero. You saw this in the dragon splat book where there was a few really good ideas in there but the majority of it was just hey here it is but bigger.
I'd rather than focus on actually doing a paradigm shift in their encounter design when you're dealing with these larger creatures so you can have those God of war type encounters. That's why I'm more of a fan of things that just typically make it more difficult for the party to rely on plan A (kill it fast) rather than just burn HP.