We all pay tax and give our tacit consent to be included in society. Some of us are fortunate enough to not need to dip in the pot much, some of us need more. We all agree on that. If we don't then we can opt out of the system and go and live in a cave somewhere.
But both people entered into a private contract with the theatre vendor for their seat. They both paid the same price. It's got absolutely nothing to do with tax/society. Or give and take. One ticket = one seat.
If an overweight person doesn't want to tackle the issue, I don't mind. It is your right and your choice. I really don't care. It's your body. I also don't mind that your obesity requires more NHS costs in the long run. Lots of lifestyle choices cost the NHS more (eg dangerous sports). I agree to live in society and pay my taxes and if some people cost more than others, that's the way it goes.
But after working hard, paying my taxes, and saving up for an expensive theatre seat out of my nett earnings I really don't need to give and take my seat because it is mine and I paid for it with my private personal resources. I don't have to share those with you. I paid for a full seat. When I booked and paid for the ticket, it was on the agreement that I would have a full seat.
My choice was to save up for the seat and enjoy the show. Sure, we'd all love a bigger seat, more legroom whatever. If I want that, then I pay for the more expensive seat. Theatre tickets, airline tickets, whatever, are absolutely nothing to do with tax and society.
Nobody makes anyone go to the shop, to buy the cookies and then forces them to eat it. One chooses to do so. The cookies don't satisfy hunger.