Is my 5 1/2-month-old eating too much?
Years ago I took care of a baby
who was exclusively breastfed but was growing fatter and fatter. At the same time, his mother was wasting away. When she expressed her milk, it quickly separated a cream layer on the top! Although breast milk is indisputably the best food for babies, breast milk varies from one woman to another. It sounds as if you are giving him good, rich milk. My sympathies for your back until he learns to walk.
The simple answer to your question is that the right amount of food is enough to keep him gaining weight and growing along his normal growth curve, which, I am sure, is way above the charts. You don't say anything about his length and about his father's and your height and weight. If you are tall and have large frames, his large size is just his genetic composition. If his development and muscle tone are normal, your pediatrician is probably not worried.
If, however, your son seems to have no check on his appetite but keeps eating as long as you keep feeding him, he may have a problem with appetite control. Make sure you give him only what he needs to avoid serious weight problems. His daily requirements at this size would be about 900 to 1000 calories. He should seem satisfied when you stop feeding him. If you are in the ballpark combining solids with an estimate of how much breast milk he is taking, and his weight gain is not accelerating, relax.