Chubby baby

When I gave birth my son was 8lb 5oz and 19 1/2 inch long. Now he is 3 1/2 months and is already wearing 6 months clothing, so he is chubby!! Every time my family or friends see him they would say “he is so chunky! Must be that breast milk” is it true breast milk make babies more plump? My husband and I are both pretty thin but when I was a baby I was chunky like my son.