The sheep used to cluster in the middle anyway. It looked bad.

The BBC says the big naked dude on a hill is overgrown and invisible, due to a lack of sheep. The caption underneath the accompanying photo says:

The giant was disguised in WWII to stop it being used as a landmark

Sadly there are no photos of said disguise, but I suspect this is close enough:

Orders: Bomb places near the enormous pagan dude with an erection.
Bombers: That dude is too villainy to be a pagan.