I’m not sure the size of the average 3 month old, but ours is a monster.
Her 2 month check-up (a month late due to changing doctors and the holidays) entailed the doctor being really impressed with her. Particularly her size and development relative to being a few weeks early. He basically said we could subtract a month in development for her and that’s what we typically could expect, but that she is really developing on a normal 3 month old schedule anyway.
Her stats: 24 1/2 inches long, 13 lbs 15.5 oz, 15 3/4 in head circ, all of which puts her in the 90th percentile for height and 95 percentile for weight. As I frequently say while pumping or watching her eat. MOO!