Hm, I personally would not do such a thing, as I am satisfied with my own penis, and if it's not good enough for someone else, they are clearly too obsessed with looks to be worth having a relationship with– and I certainly wouldn't do this for someone I'm not in a relationship with.
That said, if I had an inordinately small penis, I might consider it-- only because there is a definite limit to how small a penis can be before it become unproductive in producing pleasure for the other person, at least if while topping. I don't like to relegate myself to one role only.
If anything large penises are, in my opinion, worse than "average" ones, I can only take so much pain before it overrides and pleasure I could derive out of such a thing 
