I just remember that when I took my son in for his 6 month THAT was when the doctor said he should be able to sleep through the night (she also classified that as 6 hours). So, maybe this is too soon? I dont know that size really has anything to do with it. My son slept well early on (AFTER we got through the colic - ugh!), but my daughter took what seemed like forever to sleep all night! I don't remember exactly, but I know she was still waking up at 4 months and she also was in the 95th percentile so she was not a small baby either! LOL
Is she falling asleep while drinking the bottle? Maybe see if you can rouse her to drink more. I also would wake my babies right before I went to bed to feed him/her so I could get a stretch instead of falling asleep only to be woken up right away.
Oh! One more thing! My doc had also told us to make sure to feed that baby every 3-4 hours during the day (even if you have to wake her). She said that even very small babies (this is when DS was a newborn) will give ONE longer stretch of sleep and we wanted to train him early on to have that at night. Worked great for him. Worked pretty well with daughter. But, like I said, she was just not a great sleeper overall.
Good luck!!!