Those boxy little cars hold considerably more than you'd think.

While not a mini-Cooper, we had a Mazda 323 hatchback that carried my bass, DH's horn, my viola, us & the child in her car seat in relative comfort. Oh yeah, we tended to stop for groceries on the way home after the concert, too. It can be done.

Another thought - if you only travel with the entire group only once or twice a year, you might save money by going with the smaller car and renting something bigger for the Big Trips. (And someone else has to clean it up if the dogs do something uncivil in the back seat. ) That's what we're doing now, and I like it.