My BB is five months old now. He has only been drinking 4-5 oz for 4 months. All of a sudden, his appetize increased. He nows take 7 oz each time (4 hours in between) and sometimes 8oz.
But some BB eats even more. So no worry, as long as BB is healthy, then that's fine.
My baby turning six months old next week is naughty lately, he used to be able to drink 5oz, feed every 3 hours to recently only drink 1-2oz. It seems as soon as his mouth is wet enough to produce saliva he would stop (he likes to play wth saliva), to make him drink more we have to force him to drink by cup (but still only couple oz). These few days were record low, only drink 16-18oz per day plus half bowl of rice cereal for lunch. I am really running out of tricks as he does not like direct breastfeed (other than using me as pacifier) so I am going to try increase his cereal intake to twice per day.