If you're looking for an infant seat the Chicco Keyfit 30 is one of the smallest front to back.
If you're looking for a convertable then the Britax ClickTight line (advocate or marathon) are the smallest front to back, but are around $300. Chicco NextFit is compact, but around $280. A Graco Contender is very small front to back and only $120 at Walmart. It will for newborns, too.
Infant seat- Chicco Keyfit 30. Fit behind the passenger seat and the middle seat. Didn't have to push the passenger seat up very far either, an average adult could still sit in it.
Convertible car seat- I have the Britax Boulevard (not the ClickTight one) and it fits in middle and behind passenger just fine as well. My backup convertible is an Evenflo Sureride DLX and it fits in the middle but not behind passenger.
Re: Car seat for compact cars?