And the fact that there was a one-year time-skip in Ultimate Spider-Man after Miles's mother's death, so he had a year in-universe for puberty to kick in and give him more height and muscle, which in turn enhanced his strength. By the end of the Ultimate Universe, it had been three years since Miles was bitten by the experimental spider, two years since he first tried to become Spider-Man, and then with the eight-month time-skip between Secret Wars and All-New, All-Different Marvel, that would make it close to four years since he was first bitten. Four years can have quite the effect on an adolescent's physical maturation, especially when superpowers are thrown into the mix.