Autoflower too small

The size of your plant depends on the basic factors. Everything else is just as important as the basic factors, but these are what make the grow. The most important factors are the amount of light (watts), pot size, fertiliser, water and conditions. In your case, it is difficult to change anything now to achieve a big effect. The smartest thing to do before starting a grow is to think about what size you want it to be and then put together the required equpment based on your considerations. To answer your question halfway: you could try putting the plant in a larger pot and then fertilise the grow for the flowering phase according to the volume. The wattage of your lamp is also decisive for the yield. You can calculate this with a simple formula: [wattage of your lamp] x [0.7]. The value 0.7 stands for a less experienced grower, so the value can vary from approx. 0.5-1.5 Nevertheless, your plant looks healthy, so don't worry too much, finish the grow healthy and start the next one properly. Besides, your plant is only halfway through, so it will still grow. Happy growing!