Convertible seats generally take up less front to back room RF than an infant bucket because with older infants and toddlers you can install it far more upright than a seat can be installed for an infant. Now that the angle adjuster has been released for the radian I'm a huge fan of that RF in a small car. It's lower profile so easier to get the kid in and out of, especially in a smaller car. It also has one of the taller shells on the market, meaning it will last the child far longer both RF and FF (kids outgrow seats for height light years before hitting weight limits most of the time). The first years true fit is also a good seat, although from what I understand it's a larger seat, so may take up more space. We had two RF radians in our VW Beetle for a while and it was tight but not awful.
Britax seats do have a reputation for fitting well in smaller cars, but they also have very short shells and therefore won't last nearly as long as many other models.