I realize this may be a silly question but I don't really understand the difference between changing the size of the spot lamp in the Lamp general settings and changing it in the Spot shape. Isn't the first one redundant?
No, if you change the size of spot lamp, all shadow's edges will soften, but if you change the blend size in Spot Shape menu, it only soften lamp's. If you see the video again you'll notice that objects shadow edge isn't get blurred.
I see it like a lantern ray. If you increase the "size", that lantern will shine over a bigger area, but the light itself will get scattered, weaker. Now if you point that weak light to a cone with a 2 holes and make it come trough that cone, a bigger or smaller cone will have different effects.
Now if you turn that scattered ray into a thin, "dense ray of light and if you point that to the same cone, you get a lot more of light trough that cone. And the size of the cone also impacts.
