Such sizes makes ships behave strange. Usually when they're trying to broadside each other they're shaking like crazy. So it's more like a baseball bat now- 20+km ship trying to hit a 20m ship at a very high speed. And you can't do anything about it.
Also, I had the proto-Gigas and InfR1-Gigas fighting each other. Who won ? Noone. Because after the normal Gigas blew up the proto-Gigas, they still kept firing onto each other while having dozens of explosions. And while the debris was drifting apart, the game hasn't yet considered the ship dead, so the already-dead-proto-Gigas kept firing beam cannons and etc. out of NOTHING in empty space. It blew up the other Gigas too.
Yea... Inferno broke the record of 'Weird Ship Behavior in FS2'.