There's a leak somewhere, it seems to happen because when you start a new game as one is already running, memory usage rises more rather than falling some and then rising again (as it would if the old game world was destroyed and a new one created) - it seems that the old simulation is not being deleted when it should.
Yeah, I know. It happens if you cycle through the cars in the car select menu, too. Luckily, Linux is good about cleaning up memory after you quit the program, so it's not a huge deal. I think it's a pretty low priority, but if you want to take a crack at it, I'd be much obliged.