Oh, orks aren't nearly as big as ogryns. The average boy pales by comparison, and even nobs would be hard pressed to keep up. You'd need a full-fledged war boss to keep them on the same level, physically.

No, orks are built differently. The entire species was literally designed to be a living weapon. Not individually, but as a whole. That includes DNA encoded combat training, an almost stupidly high capacity for regeneration and healing, and the ability to produce an armed and armored army out of basically anything, and all within a frighteningly short time. That the entire species does this as part of its natural reproductive cycle is what makes orks a threat.

And, no, marines are not just 'big humans.' They're definitely well within the realm of 'super human.' The problem is that humans suck to begin with, and being a better version of something that's not very good to begin with means you're probably not all that great. The problem is that there's SO much conflicting fluff about marines, and what they can do and what they're supposedly capable of, it's almost impossible to get an accurate reading of what the average one really should be.

I mean, discounting anyone with a name grimdark enough to earn him plot armor, marines die by the hundreds on the battlefield on a regular basis. The blood angels chapter, one of the original founding chapters, was nearly brought to extinction in a battle against orks. What does that mean? Well, given that 90% of ork fighting style is 'throw wave after wave of your own men at them' (yes, they did it before Zapp Brannigan) that means that an entire chapter, known as one of if not the best at engaging in melee combat... were regularly defeated in melee combat by ork boys. The only reason the blood angels are still around is because their successor chapters had their arms twisted into coughing up enough troops to maintain the ranks.