Many value personal space on an airplane. Sure it comes at a premium but there is clearly an expectation of some personal space on airlines.

I think the public transportation analogy is flawed. The social expectation on public transport is “transport from x to y”. You don’t buy a seat on a bus. You buy locomotion. You don’t even buy a guarantee on any specific bus or subway. If the bus is full and you value personal space you wait on the next one.

On a plane it’s different. I buy a spot. Most commonly a specific spot that I can pay a premium for because I value my personal space. I paid extra for the front of the plane or for the window or for extra leg room. You are in fact paying for the space. And your paying for it on that particular plane at that particular time. Not just the locomotion from x to y, but the value that comes with knowing when you will travel, where you will travel, and how you will travel.

That said I don’t think bigger people should pay more. We all just gotta suck it up and speak up or shut up.