When I first got one of my dahlias it had big blooms, at least 4 inches diameter. Now it tends to have smaller blooms, but more of them. Is there a way to prune to make the blooms big again?
Has it been planted more than one year? If there's a huge clump of tubers competing for food in a small area you'll have lesser blooms.
Do you disbud- removing the two or more little side buds and leaving only the central one to get all the nutrients and get as large as possible? You can remove the new growth at the leaf nodes one leaf section below the buds too.
I don't know about disbranching, but someone will. I think it's done for the same reason- get the maximum nourishment to that one bloom.