I would be keen on seeing a higher number of mediums built also, I think price reduction is a good solution if handled carefully.
Would fit into historic production figures also.
Total production numbers of armoured fighting vehicles for WW2:
Nation Light Medium Heavy Total
Germany 11924 29269 8581 49774
Soviet 29770 62424 13831 106025
U.S 31187 68864 2202 102253
British (I couldnt find any easy to access figures)
Now I know its a game and running around with only historical unit quantities is not much fun, its just a guide to support what Helios is saying that lights and mediums were by far the most produced for all nations. Personally I dont believe in unit caps, I think the player should have the choice to create his own alternate history, however it should be dictated by historical cost/performance ratio. IE: I can have 2 King Tigers if i want but its going to cost me the same as 10 Pzkpfw IV (just an example)