If Giants existed in BC times...

[rdonj]

By being heavy but having very free joint movements?

I'm not suggesting that they actually get a huge defense bonus, for what it's worth, just that it would help them a lot at competing as heavy infantry. But they're not heavy infantry, they're shock infantry with ridiculous hit chances and decent damage, capable of killing light and heavy infantry alike. If you look at them in that light they're fine the way they are. The only thing they lack is a recruitable survivability unit.