There are numerous possibities, but you also have several weeks of vege growth remaining, so it should still get quite a bit larger.
it could be an auto witha short fuse, it could be environmental, it could be how you are fertilizeing etc etc... impossible to be certain from info given.
autos are out of your control in this respect - beyond benefit that results from not stepping on our own feet. nothing you can do about it. they taste liek shit more often than not, too, lol.