there is a huge difference between weighing 70kg and doing 2.5BW squat and when you weigh 100kg and want to do the same.
the latter is MUCH harder, i'm sure there was an explanation somewhere.
another example is when you see people that are like 5 ft 6 benching with a very wide grip and a barrel chest, their ROM is like 10-15cm while i have to move the bloody bar for half a meter.
Physical leverage is always going to be different I agree. A barell chested chap doing bench will always be easier than a short armed short person doing deadlifts.
However I'm conflicted, I do agree with what you're saying in your 1st paragraph, but also I agree with what Ben90 says about it just being a percentage.
However it comes down to power:weight, lighter people invariably tend to have better power:weight.
I were to shed 10% of bodyfat I'd drop to low 90s - however my strength would/should (hopefully) stay the same but suddenly my lifts become a larger percentage of my bodyweight - does it mean I become stronger? Nope not at all, however relative to my bodyweight I have. However, it may mean I can do more chins/dips as my muscles having been used to shifting my original weight will be able to do so at my lighter weight with more ease.
The way I see it (and I'm not expecting everyone to concur) is that no matter how much you weigh, lifting 200+ kg is always going to be impressive and strong. 200 or 300 or whatever, doesn't change it's constant. It'll always be 1/5 of a tonne or more as you go heavier. However, this would be in conjunction with power:weight for other exercises.
I'd never compare a 100kg man being able to DL 200kg for reps weaker than a 70kg man DLing 140kg even though they are both double bodyweight.
However a 70kg man being able to do 20 chins, will always be more impressive than a 100kg man being able to do 2.
That's where the grey area lies - it's a funny cross over.