Porsche Baby Seat?

I think I may need to post on a larger forum, but by chance, has anyone tried to use the Porsche Group I/II babyseat with a 1 year old? When I try to put Miles in the seat, he sinks behind the cushion
It says the kid must be 20 lbs and 27", but Miles is 22 lbs and 29" and seems WAY too small. Are others finding this or have I manage to do something wrong or possibly missing a piece? As it is, the $400 Porsche seat seems to be complete junk, whereas the $70 Graco works great.
