everything about Darth Vader is designed to instill fear and be menacing, from his helmet, cape, his deep electronically enhanced voice, it would make sense that the Emperor rebuilt him to have a larger and more powerful build, afterall if had the same genetics as his son, he'd be 'a little short for a stormtrooper!'
as far as Hayden appearing in the suit, I beleive it is explained in one of the featurettes on the DVD box set that they actually scaled down the suit to fit his body, and with a bit of careful lighting and camera angles, he'll appear as big and nasty as David Prowse in the original trilogy.