Thing is I do fine parrying things and it even increases our damage as anyone can get riposte damage from a shield parry.

The only reason I myself can see for using a large shield is if you are not confident in parrying (Which is fine.)

Hope they add more block to large shields cause currently upgrading them gives +5 block
But upgrading round shields gives +5 block and parry. Which might as well be +10 block.