I was the same Haroula! I was so worried DD wasn't getting enough because she wasn't drinking the whole amount in the bottle, but now that she's older she will drink a whole bottle almost every time. I think in general they just drink what they need.

There's an organic brand of formula called Bellamy's and DD loves it, maybe try that? (I think it's a bit difficult to find - i think it's only sold in Coles)