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