My baby is a tad small, too. But the normal range is 2cm either side of the number of weeks you are (so at 30 weeks the normal range for fundal height is 28-32cm). I'm at the very low end of that 'normal' range (I'm measuring 29cm but i'm 31 weeks pregnant) so although baby is small the midwife isn't too concerned because it's still within the normal range. It sounds like this is what you're experiencing, too?
I have to go back in 2 weeks and we'll be keeping an eye on it, but so long as baby is growing between visits and is no more than 2cm too small, I shouldn't think there is anything to worry about
