Let's take an example:
We have a game balanced in territory point of view.
Player A decides to spam infantry and so has 0 fuel cost. He is waiting for a call-in to take the edge
Player B has less infantry but has invested in light and med armor. he cannot make the difference because Player A infantry focus is too effective vs ligh and medium armor units.
Player A = 0 fuel upkeep. Player B = 5 fuel upkeep.
who's going to have the fuel advantage late game?
Player A with a infantry spam + Heavy tank that will rape anything on the field
Player B with less infantry, some light and medium tanks becoming useless and easy targets to the heavy. AND having to wait 5 minutes more to get his own heavy because the fuel upkeep left him behind in fuel.
To have 2 heavies on the field, you obviously need to skip light and med fuel units. And if you can do so, that's because the game doesn't reward enough med tanks users.
So make fuel units more attractive and obviously more effective and you'll solve your heavy spam problem. Nobody is going to spam heavies if he need to deploy fuel units before because it is unlikely he will have the fuel and popcap for that.
That's how was working vcoh before OF. At equal skill level, you had to build a medium tank/ tank hunter to counter other tanks. You couldn't really only rely on your Pershing/Tiger/Tiger Ace to win the game.
This is the best solution. The only way to stop Heavy tank spam is to force players to use their vehicles and Med tanks. Infrantry alone should not be able to win the mid game.