Bennets Pads are small so his arms can move around easier
According to sports.yahoo.com, it states "The Pads allow the Seattle lineman greater flexibility while also showing his pipes. Michael Bennet, at 6'4 and 274 pounds, is a large defensive end. Bennet talked to SB Nation about his unusual shoulder pad choice, revealing that they help him perfect his hand technique as a pass rusher". This shows that he is using small shoulder pads on purpose.
So in all, Bennet has tiny pads for looks, and better flexibility