I would give it another 7-10 days to fully ripen.
Without a diary and not knowing what you did or did not do, it is impossible to say why your plant is not larger.
Bad/inadequate lighting?
Heavy defoliation?
Poor nutrition/incorrect feeding?
Problems with pH?
Cheap, poor quality soil mix?
Cheap, poor quality genetics?
Poor grower skills?
Any of the above or a combination of a few of them can all lead to less than expected results.
However, autos are notorious for being poor yielders, especially for new growers.
Try photoperiod feminised seeds next grow, that way you control when flowerig occurs and you can wait until the plant is of a decent size first, rather than growing autos which flower whenever they think the time has come.