WRT leg room and space taken by the seat, there is a bit of a trade off with these, the other thing that makes a difference is recline and whether the seat is an isofix/spin seat or a belt fitted/purely RF one.
So you do tend to find in a lot of the spin seats, especially Joie, there is very poor leg room rear facing, because the spin mechanism makes the seat bulky and because these seats tend to be at an angle on the base so they are at more of an angle rear facing - if you look, the base is normally wedge shaped. This is something to do with accommodating the younger babies.
With the Swedish style seats like the Axkids, they are designed more for children 9m+ so they are very upright with not so much of a recline and you can position them closer to the back of the car to make them more compact or closer to the front of the car to give more leg room to the child.
I don't know about the pelvic fracture risk but I understand it's not considered a risk for rear facing car seats. People sometimes want to compare rear facing children's legs as being like having legs resting up on the dashboard, which is a known risk (and you should never do) but my understanding is that this is very different, probably because the dashboard is likely to have airbags in it.
I have added a couple of pictures to illustrate two things - the picture with two children in it shows the leg room difference between a typical mainland EU/UK type rear facing and forward facing seat that goes up to approx age 4 (in this case, Joie Stages or Every Stage I think) which is at the front of the picture and you can see it is quite close to the back seat so the girl has to cross her legs. The boy in the background of the picture is in an Axkid seat, Swedish style up to approx age 7, which has space for him to put his legs down between the seat and the backrest. You can also see his seat has some distance from the front passenger seat, and this isn't a gigantic car. It's hard to see the difference in the angle of these seats but the Joie seat is more reclined which is why it takes up more space.
The picture with the blue and black car seats shows the Joie i-Spin (the blue seat) which is an isofix spin seat, actually the most compact one, compared to Britax Max Way (now discontinued) which is a Swedish style up to approx age 7. The purpose of this picture was to show how compact the Max Way is compared to the spin seat. Again a smallish car.