Interesting. A bit of a departure from depictions in earlier editions. Much more nature oriented...seems they're playing up the fey aspect based on the Celtic origins of the race.
And when the PC asks why (s)he can't use the Ogre's greatclub for 2d8 when it's clearly the same weapon wielded the same way?Giant sized weapons could still work. It just shouldn't be the same as monster's giant weapons, where a second dice is added, since monsters and PC races no longer need to have symmetry,
It could be something like "when you use a larger sized weapon the dice increases by one size" or "when you use a large sized weapon you deal an additional +1 (or +2) damage".
The answer to that is "because that's the rules" or "because balance".And when the PC asks why (s)he can't use the Ogre's greatclub for 2d8 when it's clearly the same weapon wielded the same way?
While it's accurate to say there's no longer symmetry, that's because symmetry requires two different objects, whereas 5E's system has (partially) merged.
It comes down to what's more important: symmetry between monsters and PCs or player races having classical abilities?
The answer to that is "because that's the rules" or "because balance".
You can make up some handwavy explanation otherwise (not all Large weapons are sized the same, not enough mass to hit with as much force, not balanced for a creature your size) but at the end of the day you're still playing a game with rules. And a firbolg not being able to deal as much damage with an ogre club makes as much sense as the halfling fighter being able to wield the goliath barbarian's battleaxe without penalty. (If the halfling can do that, why can't the goliath weild the ogre's club for that matter?)
It comes down to what's more important: symmetry between monsters and PCs or player races having classical abilities?
Why not allow the two handers in one hand, but at a decreased damage die, just like using a versatile weapon one handed vs two handed?
Might cause problems with reach weapons, but still it shoudl be fine as a feat.
i also like the idea of powerful build treating you as large for str checks AND hiding, but I don't like the idea of shoehorning a race, so ultimately I like what they did.
Because that would be awfully complicated - there's no "natural" reduced die for each of the two handed weapons. In particular, reducing the die for Greataxe (d12) and Greatsword (2d12) without ruining the fine balance between them of reliability vs spike damage. If you make the Greataxe do d10, you'd need the greatsword to do 2d5... or 2d10/2. Awful and complicated. Or you break the linkage and then you're going to make one of them worse.
In the suggested feat, I made it affect only Strength[Athletics] because that's about the application of your strength - a little extra height and weight and some training to use it means you're a bit harder to knock down or grapple. It's uses the language that the bigger playable races get (count as one size category larger), but it provides a different effect to allow the feat to complement those races nicely.
That's not saying I think my quick feat idea is perfect, but I do think it's worth considering as a baseline