I came up with these icons to replace the current icons… well, not these specifically, but it gives an idea of what I’m thinking.

Here’s what they are, roughly:

Top row, left to right:
Medium Tank, Advanced Medium Tank, Heavy Self-Propelled AT, Heavy SP Artillery, Heavy Tank Destroyer, Heavy SP Assault Gun

Mid row, left to right:
Tankette, Light Tank, Light SP AT Gun, Light SP Assault Gun, Transport Truck, Supply Truck, Halftrack APC

Bottom row, left to right:
Light/Medium AT Gun, Light Artillery/Heavy AT Gun, Heavy Artillery, Armoured Car, Scout Halftrack

Tankette, light and medium tank icons look too similar (except for the size), as well as heavy AT/heavy arty and some SPG. It’s quite hard to tell what type of unit the enemy has if he only has one of those types (so you have nothing else to compare the size to) and you see those icons (i.e. are all of those light or medium?). You’ll have to zoom in to see what they are. So, some detail should be added that allows to tell light unit icons from medium ones, besides their size.
Otherwise those are good icons.

I’ve gone ahead and combined two ideas into one here - increasing border thickness for increasingly heavy units, and abstract symbology to represent unit role.

edit. Also, needs a heavy tank icon.

I don’t think those icons will work very well. Thin lines are seldom visible, they merely make the image look spotty. Lines must be pronounced.