Maddness of the first order. You dismiss the fluff with one hand and seek it with the other.
So if SW fluff sez that a Turbolaser has a power output of 222000 TW, yet in a game it takes 10 hits from it to destroy a 10 meter asteroid (when 1 hit should be enough to vaporize it)...which of two fluff sources is correct?
Get it now? The colli doesn't get heavily damaged because in 90% of cases the Repulse never ramms it anyway. And in the other 10% the type of damage it would recive couldn't be modeled properly anyway.
What some in-game justification?
Orion - 2100m in length (uniformed, boxy)
Aten - 230m in length (uniformed, flat)
Colossuss - 6000m in length (?? stragne volume distribution, think neck)
But we need to look at all 3 dimensions and how they scale.
The Orion is almsot 10 times as long as an Aten( let's say 9), which means it has 9x4x4 the volume (roughly) and mass, judging by the dimensions. That would come to aroun 140 times the mass of an Aten.
Aten ramming a Orion does 30% hull damage to it (roughly)
So, if an Colli is roughly 3 times the Orion's length, then it comes to about 3x3x3 the volume and mass (ideally, but due to it's shape, not quite). So 27 times the mass and volume.
So Orion get's hit by something that only a tiny fraction of it's size, gets moderately damaged.
Colli gets his by something that's roughly 1/3rd of it's size and? Minimal damage? Not bloody likely.