Every troop counts the same for happiness purposes, whether it is a low tech small troop or an end-game large troop. This is an issue with how the stock game is set up, with bigger almost always being better. For regular (combat) usage, it doesn't make any sense to build small troops when you have larger available. For policing purposes, it doesn't make any sense to build big troops. The data setup has some diametrically opposed goals.
Its quite possible to design a better system in mods, where you have actual practical differences between small troops (infantry) and large troops (main battle tanks), such as more hit points / kiloton available for infantry and more damage / kiloton available for tanks.