The size of the tummy doesn't determine how healthy the baby will be. I'm 25weeks and weigh 55.6kg, was at the gynae on Friday and the baby is at the normal weight for 26weeks and all measurements are normal. :-) don't worry yourself mommy. ..I know it's easier said than done!
Oh and I had a tiny tummy.
I don't think it really matters that your tummy is tiny. With my first I only gained 13 lbs. in total cause at the beginning I was so sick with morning sickness I lost weight. And he was born weighing a healthy 8lbs. 1oz. ;)