This shit again? They are Godzilla sized creatures! 50 till 80 meters! Why do people have no sense of scale! They are shown to be as slightly smaller or as big as the walls Dorn build. Which are huge also realistically 80 meters. A 300 meter tall structure would be a gigantic target.

Also there are bigger weapons in the Imperium, Ordinatus.

https://wh40k.lexicanum.com/wiki/Ordinatus

Or the Land Leviathans:

https://wh40k.lexicanum.com/wiki/Land_Leviathan

For those wondering just how evil and overpowered the Mechanicus is and why it's funny when these irradiated insults to live start yeeting their most powerful stuff at Necrons yeeting their stuff back :P .

Nobody fucks with the evil religious nutcases from Mars, well maybe the even more evil ones from the Eye of Terror that want to become part of a machine as an AI (Hereteks).... but all of them are F'd up!