Big freighters are themselves a problem. In a proto-trav or small-ship heresy, peventing them is a noble thing... (pun intentional)
Big ships tend to lead to centralized authority. Not something the darker traveller settings really want.
See, the big difference is that a 1000T cruiser (Bk2 or SSH) can wipe out any single 200 or even 400 Td ship all alone. But a 1000T merchant cruiser is much tougher, and requires the navy gang up. Under MT or BL, this is still quite true.
From the stand-point of creating the illusion of dominance, being able to whack any one freighter is scary for the freighter; even if 5 are the guns equivalent, if one screws up, the big ship can crunch one at a time.
HG, however, and it's TNE Equivalent BR, are critical hit systems: hits are telling if they happen at all. So the big ship is at an unrealistic advantage. If you have enogh firepower to do crits at all, the more crit-causers the faster the target dies given the same total armament pool.
T20 is a hybrid; Big ships are less tough per ton, but do far more damage per ton, plus you have "critical hits" in the above sense, as in rating reductions, not linked to size, so you can die from structure OR crits.