You might be 10 days into flower.. you have another 2-3 weeks of vege growth remaining, i'd bet.
A small plant is caused by any number of reasons. That substrate looks a bit dense - more perlite or similar would help. Also, despite urban myth, potting up should not shock a plant and you should start autoflowers in pots that are appropriate forplant size. At this point, the pot size and plant is relatively okay, but was probably in this pot to start eh? This leads to horrible watering habits and/or difficult watering methods.
The light green new growth could be a couple things. The fact it is chlorosis starting from inside out makes it a bit easier to diagnose but could still be more than one cause. Low S is possible. Or, too much P or K is also possible. A leaf chart can confirm these symptoms. REview more information about your recent behaviour to help eliminate possibilities. I doubt this is why the plant is small.
Could also be in part the genetics. Some plants just aren't too vigorous. Like i sad above, you still have ~3 weeks of vege growth remaining, so it still may reach a ~50g dry weight size, which wouldn't be too unusual for an autoflower. They are not forgiving plants.
DLI, temps, rh, etc liek others said.. lacking this info so i'm not gonna sit here and assume things... read up on DLI, vpd, and get the gist.. doesn't take much to loosely control environment, and that's usually enough for decent results.