Well for me the biggest person I have ever seen on a bike is myself. I am topping the scales at about 305 these days. I started this adventure around the 335 mark.
As a big guy myself I agree with alot of what has been said above, I think that most people have good intentions when they try to give encouragement to a big guy/gal out there riding. However the more this happens the more it could start to bother someone on a ride especially if it happened alot at once.
I run into quit a few cyclist on my rides and for the most part I just give a head nod or a wave and keep on trucking. From time to time when I make one of my stops to refill my water bottles I will run into other cyclist and they usually talk to me like any other cyclist.