Is that the kind of Imperium that GT provides? Because most systems that I've seen promote ships to be as fast as possible, and the technology supports fast ships. The M Drives tend to not be the particular limits on the design, so there's little need to restrict them. The costs of faster drives are truly incremental and minor compared to other aspects of the ships.
The primary benefit smaller ships have over larger ships is simply their size. Larger ships are easier to hit. Slowing them down makes them easier still, and in the "one shot one kill" universe of the Meson Gun "Death Ray", you want to avoid being hit at ALL above everything else.
The difference here is that in the GT ship design system, mass is tracked as well. Your M-drive produces a particular amount of thrust for its size and TL, and then our good friend Sir Isaac Newton tells us to divide thrust by mass to determine acceleration. CT doesn't track a ship's mass; instead, a given M-drive will accelerate a given VOLUME at a defined rate, no matter if it contains empty space or neutronium.
GT also allows evasion of enemy fire in a different manner than CT. In GT, your pilot rolls against [(Skill + Accel - Size-mod)/2], with a ship's Passive Defense (and the pilot's Combat Reflexes) as a modifier; if he succeeds, the shot misses. In CT/HG, agility depends on excess power being available for the M-Drive, but the Agility modifier can't be greater than the G-rating of the M-Drive; pilot skill modifies agility as [(skill -1)/2]. In essence, GT lets skill affect "penetration", and CT lets skill affect "to hit".
GT also assigns damage in terms of "damage points", and armor reduces the effect of every shot that hits by its Damage Resistance (except for meson weapons, although Meson Screens provide armor against those if the operator makes his skill roll). Spinal meson guns typically do so much damage they'll get multiple Major Damage rolls against anything smaller than 100,000 dtons; those are roughly equivalent to HG's "Critical Hits", but a 500 kton DN has so many hit points that you're not going to kill it with a single shot from any weapon in the Traveller arsenal.
Under the GT ship design system, a
Tigress is big and slow (with a 2-G acceleration and a +15 size-modifier), but it'll take anything you can dish out for a very very long time.
However, I'll also point out that acceleration is just one factor, and that initial velocity is going to be very important, too. The GT version of the
Tigress doesn't change speeds all that quickly, especially when compared to a 6G-capable ship, but once it gets up some speed, it can move along with alacrity. Not a lot of fleet actions will be decided in a single pass of ships, anyway.