Issue - micro plants - mini harvest - why??

My advice.....don't grow autos, or at least not until your technique is dialed in. For many they are an attractive option since they sound easier since they're not light dependent but that aspect actually makes them harder to work with because any stunting is hard to bounce back from. A photoperiod plant is much more forgiving since you hold the control on when it switches cycles.

Those don't look like "4" gallon pots. They look like 2-3gallon tops. Pot size does affect an autoflower in significant ways. If the nutes are not 100% chelated and plant-available when dissolved, that too can be much slower gowth. However, if you are running that light anywhere at 100% over 18h from roughly 18" give or take depends a lot on physical shape of the light, You've probably just stunted them with light in addition to any other potential factors. Just using rounded numbers, but from spec sheet-- Your light produces 500 umol/s. In a .64m^2 you need 600PPFD over 18h to reach 38.9DLI. So, 600*.64 = 384 umol/s to reach 39DLI. 380 / 500 x 100% = 76%. If running at 100% you are giving 133% of what that plant can handle per day, conservatively. Likely a bit more, as 39DLI is probably overshooting slightly, but I am purposefully erring on the high-side slightly. 35 DLI might be closer to all you can give per day with ambient CO2. 39 * 133 = 50+ DLI, holy shit, batma! you were frying the shit out of those plants, that's why. This might be a good range for 1300ppm of CO2, if supplemented. Also, environmental variables may allow for one garden to handle more DLI than another... temps, rh, properly feeding, etc... So, again, this is a ballpark concept and requires some observation and adjustment, but they will be small adjustments / easy to recognize. Even "39" might be 10% too high, but i err on high side for many reason.. can always dial back a bit but you can't manifest more light from nothing. In the future, try 75% with properly matured plants is a good starting point and observe/react. Probably need to dial back as much as 10% of 75% -- which i mean -7.5% and not -10%. This time, however, you may want to start at 65% and err on safe side, then, if any miraculous stretch occurs, you can adjust based on resulting growth pattern. The longer they have been pelted with too much light the longer it'll take them to recoever, which at this point may or may not happen. They may just be hopelessly stunted for good. All you can do is try. Think you just stunted them with light. Would expect a bit tighter node spacing, but if you were giving 100% from a typical distance, this is undoubtedly a major reason for the resulting stature. in future use the resulting internode length to fine-tune power of light. too long, it needs more. too short, it needs less. An internode is simply the length of stem between 2 nodes.