I posted a picture yesterday of Lady in her first hoodie. I decided to send it back for one size bigger. It is too tight on her. Lady is really broad in the chest which makes fitting difficult. I think yours fits just right. There is a little wiggle room in it. Looks cute!
Have you washed and dried it yet? They do shrink a lil after that. How much does Miya weigh? Izzie, who is just under 4 lbs, wears the XS, and Bella, who is 8.5 lbs, wears the small. Here is a thread with pics when my girls first got theirs.. It shrunk up a little so now their butts show more. Also they have gotten a lil thicker since those pics. But I can still easily fit their harnesses under each sweater.