Actually the problem with them being bigger is:
-The pathing gets MUCH MUCH crappier
-Less units to gather around buildings and attack. (This is why portable rams are quite strong as its very easy to put 10 rams next to a tc and attack than 10 siege towers)
-Easy for units to surround the giants and slaughter it.
"Truely, if there is an evil in this world, it lies within the heart of mankind."
"I'm going to live forever or die trying"