18 weeks pregnant and baby messured 11 inches?

I'm almost 19 weeks now, i had my 18 week altra sound last week. My baby messured 11 inches from head to butt. I didn't think much of it until other woman who are bigger than me I haven't gained alot of weight either and just starting to show. I went through the same thing with my first, i stayed small until my 5th-7 month then i look like i ate a whole meal plus. I feel these woman are giving me a hard time, I don't think they realize every woman is deff just like every baby is deff and if my ob was worried she say something. I work-out eat healthy take my vitiems and make sure to take care of myself. SHould i'd be worried about my baby? being 11 inches? or are these other woman ful of themselves? Oh i'm only 5'0 and petiet size, i've only gained 10 pounds but it went to my breast and butt lol, I just started showing.