My baby have the same situation since she was 2 months. Before that , she can eat 6 ozs each time , but after she was 2 months, she only eats around 3 ozs. Until now, she still eats 3-4 ozs each time, so I started feeding her rice cereal when she was 4 months. I hope she can eat more.
She was 5.5kg (12.2 lbs) when she was 4 months. Very small baby size. But I can do nothing as if I force her to drink, she just cries or even vomit.