The way to measure frame flex is to stand facing the bike holding the handlebar and seat. With the pedal facing you down, push down with your foot and see how much the frame flexes. You will be surprised. Rider weight is an interesting metric but it is power that flexes the bike frame. 140 pound pros generate a couple of thousand watts where a recreational "heavy" rider may generate a few hundred when standing. It is the pushing down and pulling up with both the feet and hand that matter.

Yes, wider tires can be more flexible when you have a less then perfect road, I have a 25 on the road bike and that's fine on anything from chipseal on up, the MTB has 26x1.75's (44.5mm) on it and those are much better on gravel or dirt, I would love to try a 32 on the road bike, but the fit would be iffy, even though it had 27" x 1¼ on it when new. I think the tire would fit if it was flat, then air up once it's installed.