Possibly relevant: Something that is currently thought to maybe help with the insulin-resistance related issues around PCOS (weight gain and too high level of androgens, because insulin that sticks around too long gets converted to an androgen [can't remember if testosterone or not]) is high doses of myo-inositol, which naturally occurs in fruit. So maybe the watermelon is helping with regulating your insulin through the inositol (in addition to or instead of the citruilline) and if there's less insulin in your blood you crave sugar less?
PCOS and insulin resistance vs. what that does with metabolism etc. is a whole other can of worms, but it seems like the research into it might at least have some overlap with what you'd like to figure out.
Not sure, I (potentially) took it as part of a study while pregnant, can't really say if it had any effect because pregnancy causes so many changes by itself (if I even took it, of course). That was 4g a day (combined with 0.4mg folic acid), by the way, which is roughly what they suggest for PCOS in general. So your supplement + watermelon inositol dosage does seem low in comparison, but maybe the 4g is higher than it needs to be, it's hardly settled science. And as you say, threshold effects or a combination of things, or simply that you don't have PCOS so you need less of it to achieve a similar effect, because you were also less dysregulated. (Why it would be dysregulated in your case or in PCOS patients may or may not be the same, no idea in either case what the cause is, gah.)
Interesting, I didn't know inositol had so many potential applications, I wonder to what extent these issues have a similar root cause in that case. At least they seem to have a symptom in common.