I had duckweed in two tanks, first one was infected with it, then the other was contaminated. I say contaminated because I cant get rid of the stuff and to indicate that the duckweed came from the same source and is of the same germline.
Anyway, both tanks have identical water conditions, both get EI dosing, but one tank is only a single t5ho low tech and the other is a dual t5ho + CO2.
The low tech tank always had larger leaved duckweed than the CO2 tank. The CO2 tank was almost microscopic in size. (Well, maybe I exaggerate). THe important point is that they are different sizes.
So it is either the extra light or the CO2 that is stunting their growth, but the size difference is quite noticeable.