The notion that Randy Couture "has troubling fighting 'big guys'" is nonsense at this point and should be dismissed by anyone who promotes this falsehood. To argue that Couture has unique difficulty fighting larger opposition more so than other fighters in similar circumstances has no evidentiary backing. The truth is that the situation is too complex to be neatly summarized or packaged in such a glib generalization.
First, what defines "big" in this case? Men larger than him? Men larger than him by a significant margin? If so, what's the margin? What people are driving at when they say "big" in this instance is that the weight disparity between Couture and whomever he is fighting is significant. Rather than two fighters meeting at the same weight at the weigh-in and then blowing up at marginally different levels, Couture has often conceded 20 or more pounds to opposition. But aside from a very, very small subset of elite fighters, who doesn't have trouble with men significantly larger than themselves? That's a rule of thumb that applies to virtually everyone in the fight game, not simply Randy Couture. In fact, that's the entire reason for having weight classes in the first place: disparities in weight unfavorably stack the deck for the heavier fighter. The heavyweight class offers the most room to skirt around that concern, so Couture has fought and lost to fighters significantly larger than himself. But that doesn't equate to Randy having the unique problem of handling size disparity, either in light of his recent record or why he's different in that regard from other fighters.
Second, Couture's return to heavyweight saw him dismantle two very large opponents with different sizes and strengths. Sylvia was ostensibly the better striker of the two while Gonzaga the better submission expert. In both cases, Couture was able to work around their size to implement his gameplan. That means since 2002 Couture is 2 - 2 against larger opposition in the heavyweight division and has wins over two different kinds of larger men. If the theory is that Couture has trouble against "big guys", shouldn't that theory hold up across a range of styles and body types? And disregarding the problem that Couture won both of those fights, he didn't even struggle in any sort of similar way to what caused his losses to Barnett and Rodriguez. So what is the common theme here? There was nothing in the Sylvia and Gonzaga fights that can be retraced to the Barnett, Rodriguez losses that demonstrates Randy having the unique problem of handling size disparities.
Admittedly, when this bromide was issued Couture had lost to Inoue, Overeem, Rodriguez and Barnett. But Couture would undoubtedly smash Overeem, Rodriguez, and Inoue now. Barnett would still prove a very difficult test, but the notion that Couture has difficulty with physically larger opponents weakens further when we are forced to place caveats on those heavyweight characteristics. Does he lose to larger men or larger men who can bang? Does he lose to larger men or just larger men who are good ground technicians? The theory is beginning to be whittled down into non-existence when those considerations are included.
This isn't to suggest anything about the outcome of the fight. Rather, I would ask that we stop promoting the idea that fighting larger opposition is something uniquely difficult for Couture. The truth is that fighting larger opposition is uniquely difficult for anyone, so the theory is already dubious. When we further include Couture's success in his last two fights, the theory ceases to be even remotely meaningful. There is nothing in Couture's current skill set or modern record that suggests he uniquely has difficulty fighting larger opposition over any other fighter in a similar circumstance.
It's possible that Lesnar's size will be a deciding factor in this bout. However, the rule of Fighter A having an advantage of 50lbs lighter Fighter B applies in virtually every circumstance. It won't always be the case that the heavier fighter's size makes the difference (see Fedor vs. Hong Man Choi, a true example of diminishing returns), but in the overwhelming majority of cases the heavier fighter enters the bout with a significant edge. That Couture has come up short against much larger opposition while also dominating them makes the picture inconclusive at best.
