Milk intake for my 10week old?

Kara
Hey all!
My 10 week old, who was born 3 weeks early (so 7 weeks gestation age), still only takes an average of 2 ounces of expressed milk per feeding. The most she's done is 4 but that is very rare. She also is a lazy & slow eater. Never really "chugs" a bottle. 
On the breast, she now is pretty efficient and does about 5-8 min on each side. 
However, she is measuring super petite (8th % for her birth age, 20% gestational age) 
Do you guys think I should be worried that her intake isn't that much? She's feeding every 1-2 hours during the day and sleeping thru the night. 
Thanks!