I think part of it is that you draw Digitigrade legs a lot, which requires a shorter leg, which is made up for by the extended "foot". But with Plantigrade, I was always taught they need to be longer. Admittedly, I use more "comic" and "anime" lengths, which actually exaggerates the length of the leg a little bit, but it does seem like you're making the legs a bit on the short side. The measurement I always use personally is the length from shoulder to hip is the same from hip to knee. That exaggerates the length of the leg, but only slightly. Then of course the length from hip to knee should be the same (in plantigrade charas) as from knee to ankle.