Well, given the simple fact that it takes much more blood to fill a very large penis than an average/small one, it would not be surprising if they function a little differently.
Here's what I have observed with my 11"x6" penis: I have never had what I consider a "problem" with erection in the sense that I have always been able to get and stay hard enough for sex and to achieve rock hard firmness (though maybe still retaining a little more flexibility than a small penis would). But, as long as I can remember, while my penis will sometimes swell to semi-erect on its own, it usually takes at least a minute or two and some physical stimulation to reach full hardness and continued stimulation to keep it at that level. By contrast, I hear of "normal" size guys just springing instant stiffies that stay that way without any effort. (As a straight guy, the only other erections I normally see in person are in the gym showers/locker room. But I have seen more than a few of these instant boners pop up when I'm in there. Coincidence? Maybe.
) Also, girlfriends have commented that it seems to take more time and effort to get my dick up to full hardness than their past, "normal"-sized lovers.
A few years back, my doctor diagnosed me with high cholesterol. Because of my age and relatively healthy lifestyle, he said it must be hereditary and put me on a statin drug. Much to my surprise, once my cholesterol went down, I noticed a significant improvement in my erections (both speed in getting erect and overall firmness). Again, I did not have a real "problem" before, but I think the cholesterol issue was affecting my erections gradually over a few years before it was diagnosed, so they are better now than they were before. Great incentive to watch the cholesterol!
Finally, I also notice even better erections when I am taking L-Arginine supplements. I started taking those for bodybuilding purposes, but started noticing the side effect that my boners seemed even firmer and faster. Did some Googling and turns out this is a well-known benefit of Arginine. (Makes sense as the stuff works by increasing nitric oxide, which increases bloodflow and is an important part of the whole erection process.)
So, my erections were always fine, but between getting my cholesterol down and the Arginine supplements, they are better than ever. Maybe large penises are more sensitive to these things because they depend on getting more blood than smaller penises. I don't know. But my advice if you're having erection issues is get your cholesterol checked and try Arginine supplements.