(Back story: My five week old nursling is a big baby and growing fast, I have a large milk supply, and I wonder if he is getting too much milk from me.)
I forgot to mention: my first baby was very big (and I did not have a super abundant milk supply). He was consistently in the 140th percentile on the growth chart, super chubby, and exclusively breastfed. My doctor was always pleased that he was so roly-poly, and would take him around to the nurses saying, "See? THIS is what an EBF baby looks like!" (My second was a skinny baby and nursed even more than my first, so I just chalked their body differences to genetics. Both are normal, as long as they're growing steadily.)
But see, if a baby's tummy is uncomfortably full, it will latch off and/or spit up. Babies are instinctive nursers, which means they're not like adults who overeat for emotional issues or other reasons. When they've had enough, they stop.
I think you can cross this off your list of things to worry about. A baby who's growing well is a good thing! :)
Our guy was 7 pounds 12 ounces at birth and never lost any weight and I had oversupply and he was an avid nurser...he gained quickly and at 4 months was in the 95thish percentiles for height and weight. Then at about 6 months his growth slowed considerably and by 9 months he found his true growth curve, 50th (weight) and 75th (height) percentiles--where he has stayed. Breast fed babies "tend" to grow quickly and as their needs change breast milk composition and the manner in which they nurse changes...so my still in the 50th/75th percentile nursling (19 months) who nurses around 5 times a day and is a fairly avid eater of solids really did grow fast at first but then slowed WAY down.
believe me, if a baby doesn't want milk, he won't drink it. i don't have time to find the link right now, but normal breastfeeding patterns in infants are to nurse very frequently - my DS was most definitely latched more than unlatched during the first few months (having a sling GREATLY improved my ability to do anything rather than just sit and nurse....although I did my fair share of that).
remember that nature is smart, and that our bodies adapt to the cues that we are getting and babies are constantly giving cues. if you are pumping you will notice that sometimes your milk will be very watery, and other times it will be very creamy, and others in between. the consistency changes to fit a baby's needs - both during the day and through out each feeding. If your baby is gaining well, then you don't have anything to worry about.
like a pp, my baby only lost 4 ounces when we took him home from the hospital, and grew off the charts very quickly. he didn't even "level off" by six months, and is now 29.5lbs at one. doctor is very pleased.