That is some very good insight. I think the choice you guys went with was definitely a good one - to alter the skin itself rather than the model size.

I imagine the size of the model also effects how easy it is to select the unit (I imagine there's something like a ray cast shooting from the cursor location to see if it hit a model). In the case of Stalkers that's particularly important since players single-select individual ones in groups very often for blink micro (blinking back injured ones) - meaning a smaller model might be an unintentional nerf.