Speculating further on the issue of High Guard verses the canon fleet, I see four options:
4. Assume some third factor is involved. Perhaps fleet ships factor in ammunition, though High Guard does not. Perhaps the big ships' recycling systems benefit from economies of scale, with on-board waste recycling and food synthesis facilities allowing for longer deployments away from supporting bases and ports. Perhaps the Emperor's just got a fascination with big ships.
IMHO there's a third factor involved, just not commonly used: crew quality (skills).
Probably your fleet has a handful of individuals that are Ship Tactics 5 and another handful that are Pilot 5.
If you concentrate them on very powerful ships (let's say your Tigris), they suddenly have +2 to computer size and +2 to agility, so making your gebil/hamster quite less powerful (they hit you on a 10+: base 2, DM +2 for size, -8 for modified agility, -2 per modified computer size), and penetrate your nuclear damper on a 12 (base 10, DM -2 per modified computer size.
In the meanwhile, your Tigris will be using its own missile batteries (with its +2 computer size) against your gebil/hamsters to quite good effect (to hit is 6+: base 2, DM +2 per modified computer size , -6 per agility and penetrate even a factor 9 nuclear damper on a 8+.
Sure, some of you gebil/hamsters will have similar officers, but they are only a handful, so greatly reducing the numbers advantage (and probably losing them quite more often that those on the Tigris)...