Should there be a weight limit for airline seats?

I ask this question b/c two out of my last three flights I have had the pleasure of sitting next to individuals who had SEVERE issues when it came time to shimmy their bums into their seats.

Is it fair to ask people, like myself, to sit next to an individual who clearly does not fit in their seat?

Moreover, is it fair to ask passengers to "weigh in" for their particular seat purchase?

I am torn.

I am sick of sitting next to individuals who take of 50% of my seat, yet I somewhat empathize with those whose struggle with their weight and would be singled out if such an airline policy existed....