There actually are posts on these forums from women remarking on this very issue. One of the consequences of fat loss being out of our direct control is that it disappears from the oddest places, and in apparently random order, sometimes.
@Gary1209 As for the topic of this thread, I can’t think of any reason why a ketogenic diet or fasting would cause this problem. If it were a common result of fasting, men would not fast, lol! Also, fat is not generally stored in the penis, so I can’t imagine fat loss affecting penis size. As I understand matters, the size of the erect penis is related to the frequency of erections, and that is the result of a number of factors.
One such factor is, I believe, your testosterone level, so you might want to get that checked. Also if you are taking a statin, or other cholesterol-lowering drug, remember that not only do statins diminish the frequency of erections, but also testosterone is made directly from cholesterol, so lowering cholesterol could be part of your problem. Plenty of saturated fat in the diet will help with your cholesterol, and hence your testosterone, levels.
In any case, this sounds like something you should be discussing with your physician. I doubt we can help you here.