Sometimes you will get phenos that will remain small throughout life (runts), they seem the be few and far between though, but I have had the same issue with other plants autos and photos alike...
The other, and more common reason plants stay small is that they were stunted because of small containers and late transplants, basically grower error. It isn't hard to do with autos, just restrict the growth of the first big taproot and the plant will stay small even if transplanted later...
Was the seed you started this with put in the container it is in now to germinate? Or did you do any transplants or paper towel germing?