I think that Cav's "2nd kick" isn't what it appears, after listening to him on Talksport late last week he explained a lot about the sprinters.
He can "only" produce a peak of around 1200watts - and thats mostly in training - Greipel can do over 1700watts. Cav however can maintain his peak for 15 seconds and the others, Greiple included about 2-3 seconds. So I think that his 2nd kick may just be the others deccelerating, not him accelerating.
He's also not that small in real world, let alone the cycling world at 5'9", he just appears that way as he's quite stocky for a cyclist. Sagan is 6ft and only weighs 7lbs more.