Ian, thanks for the insight. Full harness seems to be an odd duck; while I agree a marathon runner with slim calves would certainly not be out of shape, I do wonder how long they would be mobile in harness with all the extra weight. And if they were moving about in that weight, wouldn't it increase the size of their calves at least somewhat? They could have slim calves but be considerably strong; comparing bodybuilders to some of the "world's strongest X" and the "tough man" competitions, some of the most ridiculously strong and brute-force guys have a very moderate build compared to a "body builder" - so I try not to equate mass with power - there's some correlation, but it's not the end-all, be-all.
Anyways, I mentioned the 'metal cast' (not Ivan), since it's the closest way I could think to describe them. It seems paradoxical, but I'm finding with large calves - or rather, a large variance between calf & ankle circumference - that the weight of the greave really *needs* to sit over top of the foot to allow slight play at the calf, or I can't kneel / kick / hop around comfortably and with full range of motion. For reference, my calf is 16.5" (sadly, it's shrank a bit), and my ankle is 9.5". That's with no exercising. When I was active and in shape, my calf was 18", nearly double my ankle.
The large difference in measurements like that makes it really uncomfortable for the back of the greave to be perfectly fitted to my leg; I keep it as tight as possible against the front (as you could probably guess from my armor bite photos), and let the back have a bit more play. This lets my calf "flex" up and down my leg as I squat / kick / move about. If the back plate is resting on my calf for support, to keep it off the front of my foot, it's horribly uncomfortable on my calf to do any movements that cause my calf muscle to move - I'm literally pressing my muscle up against the steel. With the weight from the greaves supported by the sabatons, and 'separated' by sitting on the spur straps, it's completely comfortable.
I can't find a decent picture online, but very very late period greaves were sometimes solid from the demi-greave down to the bottom of the foot. I don't know if that's to keep the weight over top of the feet like I'm doing, because they were worn on horseback and the stirrups would be taking the weight, or some other reason.
I think the smaller calves makes it easier to wear greaves since there's not as much muscle mass moving around, and tightly fitted armor can be self-supporting without being restrictive. If I just had schynbalds, I don't think they would need to sit on my feet at all. It's the motion of the large calf muscle that complicates it for me.
(Sir Edward - can we get a topic split to something like 'Cased greaves & Sabatons' with some of these posts? I've derailed this thread rather far. )