I'm skinny and small and still find seats on planes really tight and close.

Yes, the egalitarian politics of the day greatly inconvenience those with a more modest...footprint.
Shame about that. Volumetric (or dimensional) weight is standard for logistics more generally. I think we should use it for people. Makes a great deal of sense.
And this doesn't benefit me personally. Severely malnourished as a child, I grew to average height but got the frame suitable to a taller body. I'm built like a wrestler, and dropping below 170lb is not workable.
But someone with a small build, and body weight closer to 100lb, should really pay less for a flight than I do. It's very logical.

People who exceed a certain dimensional weight should pay a surcharge for separate seating, where they won't spill into someone else's personal space. Because having that happen to me on a flight to Europe was no fun at all, and furthermore, speaking of snoring and loudness in general... Excessive noise and disruptive physicality should likewise draw down a "deposit", to be distributed among the affected passengers around that location. Something for their trouble.
Children should be charged as adults. No exceptions. You know why.
And lastly, can we PLEASE have a quarantine section for the ill and/or infectious? I'm not here to be coughed and sneezed at.

Taking up space as a fat person can be a huge source of anxiety, due to attitudes that their bodies are not allowed/valid in the same way a thin person's body is. Take a minute to read this open letter, and think about it...
https://medium.com/s/for-the-record/a-letter-from-the-fat-person-on-your...