A bit worried

Hi, theres something ive been worrying about a bit. Im 16. A couple weeks ago i measured my size and it was pretty good in my opinion. Ever since then, even when i feel fully hard the size isnt as big as that one time. Im sure i used the correct way of measuring that time i got my largest measurement, so why was it a one time thing? I feel like i still got growth left, but this still worries me.