I think Hulking is a bit of a mixture between size and sluggishness (you might want to call this mass to muscle ratio or something similar sciency but for me its mostly a 'what feels right' kind of thing). Once you get enough HUGE POINTS you become HUGE (hulking), but if you only barely scratch the limits between hulking and normal sized its your sluggishness that might make the difference.
I think a naked Space Marine would be a bit smaller than an Ogyrn.
A Space Marine in Power Armour would probably be about the same size as an Ogyrn but due to the SMs Black Carapace they don't become as sluggish as a human would from the Power Armour so they only become Size(Hulking) without the penalty of becoming easier to hit.

Space Marines are somewhere between 2'10 and 2'5m in height, while Ogryns and Ork Nobz are easily 3m tall, maybe more. TT minis are not entirely to scale, but still a Nob is huge and an Ogryn is even larger. Power Armour doesn't make you that large, but for people without a Black Carapace, it is bulky, uncomfortable and slightly sluggish, which would justify the penalties. 

JuankiMan said:

Entirely agree with this line of reasoning. A marine in PA treats it like a second skin, so can evade and shrug off damage better.

Kasatka said:

See, if it's supposed to work that way, the existing mechanic does a bad job at representing it. In my book, you'd actually need to be aware of an attack to attempt evading it. Again: Why is it harder to hit an unaware Space Marine in power armour than an unaware human in power armour? The funny thing is, the Marine should be even easier to hit under these circumstances since he's larger still. Instead, the opposite is the case.

If PA reduces Agility (which does not only affect evasion), it should come with exactly that as a penalty - and have the Black Carapace negate it.

Alas, I fear such changes would cut too deep into the game's core mechanics.

I think one way to think about it is that marines tend to carry themselves in their power armour in a way that is distinctly different from a mere mortal. Even passively, while unaware of any incoming attack, they move with such a grace while still being giant, to the point that they might as well be "normal" sized.

Honestly, I feel the exact purpose of the Black Carapace as it is written in the rules is to aid the GM's sanity.