I like the logic of getting less from immature plants. The pedagogic problem is how to make the player aware of it so it's not reported (or just perceived) as a bug over and over again. The sage might possibly say something to that effect when giving the quest, or the quest might have a "could you give me some advice?" option so you would read it only the first few times.
Something along the lines of: "As I'm sure you know, mature plants can be picked and then threshed to provide the maximum yield, but both immature and mature plants can also be relieved of their leaves by simply picking the leaves, leaving a picked plant behind. As immature plants have smaller leaves than mature ones, you would need to pick more of those to get the same amount of leaves as you would get from mature plants."
Even better, of course, if the advice would be plant specific, such as where you'd find the plant requested, but that's also more work.