I tend to ignore that High Guard-ish stuff. In my game I've statted up, and the PCs have seen in the distance, a 5,000 ton Imperial Dreadnaught and a 5,000 ton Imperial Carrier with abut 90 fighters on board. (Both of those are of my own design. I also worked out my own rules for bay weapons and spinal mounts, broadly inspired by High Guard but roughly balanced with the Book 2 turret-mounted weapons.)the REAL Imperial navy ships mass up in the 100k ton and up category, sport terawatt energy weapons, 1000's of turrets, bay weapons, etc.
I used bits of the High Guard rules to design a tens-of-thousands of tons space station, but don't think those big warships add much to the game.