To put it more simply - The small balloon is smaller, it doesn't just look smaller because it has less air in it. The air in it is under greater pressure than the air in the big balloon, so it squirts into the big balloon.
Thanks for the clarification there Sid. Yes, my (ex-)mechanic is indeed a crook. As are most mechanics, in my experience. Not a problem for me any more as I've ditched the car and it's ever-increasing running costs.
I let mine run out of fuel once, and it's never run right since, even after having the fuel system cleaned out and repressurised.