Just got back from my 20 week u/s and i have twins (fraternal). ONe mesured 12 oz and the other measured 9 1/2 oz. The palcenta on the smaller baby is measuring smaller too.
They can't see any obvious genetic problem or physiological problem on smaller baby, so worst case is something is very wrong with her OR best case is that she is just a little peanut compared to big sis.
As I've been told by us tech it is ok for babies to measure differently. It is two different babies after all. Mine are measuring differenly as well. Right now, I am 25 weeks, baby A measures 1 lb 12 oz and baby B measures 1 lb 9 oz. Try not to worry. IF doctor didn't see any problems then it is ok.