What's proper taxi selection protocol in the following situation?

Our group of two couples were leaving our hotel in Barcelona at 3:30am to catch an early flight. There were a handful of taxis parked beside our hotels with drivers in them waiting. The first car in line was pretty small but likely can stuff us in for a relatively uncomfortable ride to the airport. The second taxi was slightly larger and correspondingly more comfortable ride.

We took jumped in the first taxi and while the ride wasn't the most comfortable having to bring some of our backpacks with us to our seats instead of squishing them in the trunk.

How obligated were we to take the first taxi over the second one? Or is it just a free for all because of our space requirements?