3rd child, upgrade car or downsize car seats?

I am pregnant with our third and last baby. We currently have a 2015 Prius. My daughter is 12 and son is going on 3. So with the baby there will be 2 car seats and a pre teen shoved back there. We talked about getting a bigger car, but after doing some research and walking a couple car lots we decided to downsize the car seats instead of going 15-20k more in auto loan debt..

Larger families, what did you do? I know we could find a cheap van, but having a hybrid car is important to us, both for the environment, street parking and fuel costs in the city.

So far we swapped up our sons graco 4 ever (great car seat, just super bulky and HEAVY, very hard to move between cars) to a graco tranzitions.... what are some smaller infant car seats?

Luckily they make more streamlined car seats which are just as/more safe as the oversized seats. As was our last toddler seat, some seats are GIGANTIC. Or as my husband said, "Joaquin has the Cadillac of car seats..". Here he is in his "big boy" seat, which he loves!!