Girls hit puberty sooner than boys, and so get their growth spurt early. It may be that many of the 11 year-old girls were bigger and stronger than their male counterparts.
Around 5th grade, it’s not unusual to see a lot of girls who are taller than most of the boys. Just a few years later, almost all the boys are much bigger and stronger.
At 11, it’s not that strange to see girls who play Little League baseball or Pee Wee football as well as the boys. But by 13 or 14, almost none of them can keep up with the boys.