After having numerous scans with both my boys pg's (mainly DS#2) it is common for sizes to vary due to the position of the baby and i think the skill level of the person doing the u/s. DS#2's measurements kept fluctuating throughout my pg so don't be alarmed, just like out in the real world bubs will grow at his or her own rate whilst in the tum. I was told that as long as they were within a certain range not to worry as one way or another they will catch up. As both of my boys were small (5lb,9 and 5lb,5) DS#1 is now a good size against his peers who were full term/higher birth weight.
I wish you all the best for the remainder of your pg!