Sounds to me like she is ok. If she is eating solids fine, nusing and having formula and gaining weight - I wouldnt worry either.
I went throught this whole thing where I thought my dd should be eating more than she was and then I was told how little her stomach actually was. She was eating appropriately. (I still wonder occasionally)
What is your letdown like? Is it immediate? At 5 -6 mos, your dd has established your milk supply to a point were she can get a lot of milk in the matter of minutes. At that point they are efficient eaters.
Also at 6 mos she could be going through another growth spurt. (It might explain the frequent mini nursing sessions)
How many ounces is she drinking? How do you know she isnt getting enough ounces if you are still BFing?? (there is no way to measure it in ounces unless she is getting BM in bottles only)
Breastfed babies eat only what they need when they need it. They are different than formula fed babies in the fact that a formula baby will eat an 8 ounce bottle all at once and a breastfed baby will have a sip here and there and then nurse for two hours.
Is your baby having enough wet/dirty diapers?
Check out this website... maybe it will have some answers for you.... http://www.kellymom.com/bf/pumping/milkcalc.html