The first time I expressed my son was around 8 weeks, I got 8 ounces from one side (I was full and i make a lot!). Even then my son could take a whole full breast if he wanted to, meaning he was getting around 8 ounces a feed at 8 weeks. I worried like you that he could be getting too much, but was reassured by health visitors that he cannot get too much breastmilk.
I can still get 8 ounces plus from full boobs (not that they're full very often with my piggy!!!), but how much my son takes can vary as to whether he's having big feeds one day or snacking! Even now (hes 14 weeks) if he has an expressed bottle I don't give him more than 4 ounces as I don't know when he has more or less in the day only he does and he can't tell me. So he normally has 4 ounces and i top him up with the breast if he wants more. If I was to leave him with someone I would give him 4 ounces in a first bottle but leave more in another in case he wanted more.
Basically he can't have too much, breastmilk is digested much much faster than formula so they do take more.
Becki x