Sac larger than normal........

So I had my first US @ 7 wks 3 days on 6/8 ...... my doctor called and said everything looks great, good strong heartbeat of 155 BPM & that the baby was measuring up exactly what it should be from the last time I had my period, she said there is just a minor discrepancy ..... the sac the baby is in is a little larger than normal..... not to be worried that they only get worried when it’s smaller but just to be sure do another US in 1-2 wks..... so I have it scheduled but I can’t help but worry bc why would they want another US? ..... anybody else experience this?