Ok, let’s say you had a coworker you have been carpooling with for a while; their weight is starting to ruin your seats. How do you tell them they are too fat to ride in your car?
You don’t.
Agree with Baker. You don’t.
To echo Baker: If you’re looking for a resolution to this issue that leaves feelings unhurt and you in everybody’s good graces, you’re not going to find it.
Your best bet, quite frankly, if you do not think you can be up-front with your coworker, is to make plausible excuses as to why you can no longer carpool. Show up at work an hour earlier, take your kids to school on the way to work, etc - whatever works and is a somewhat socially acceptable excuse will do it. Telling your coworker face-to-face that “your weight is ruining my seats” will not go over well unless your coworker is in fact secretly a robot in a very convincing human suit.
Ok, how can you save your seats before they get trashed then?
EDIT: I read the post above this one. Its one of my friends coworkers.
But in my opinion you shouldn’t do any of that merely out of pretense. You should just accept it out of human kindness and chalk it up to reasonable wear.
Agreed that the best option is simply to write off the seats - but if OP is adamant about saving his/her seats, anything but telling the coworker in question to their face is the answer.
Edit: I seem to have quoted the post right as Acsenray edited it. Please note that I have not in any way attempted to alter or distort the content of his post, and kindly refer to the post immediately above this one for the accurate quote.
No worries. Dang autocorrect.
This is actually a thing?? Fat people ruin car seats? Really?
Ask them to walk backwards and make a beep beep beep sound. That should clue them in. 
Make them ride in a trailer.
I would never ever tell someone they were too large for the car. I don’t think my friend would either. Thanks for your thoughts guys.
Surreptitiously shorten your seat-belts. “Well, I can’t let you ride! It’s against the law…”
(Okay, no. But I do have an obese friend who has to wear the seat-belts uncomfortably tight. I have seriously thought about trying to find a seat-belt extender.)
Get a motorcycle.
I have to ask the same question. How big is this person that a steel framed, foam padded car seat is getting “ruined”. Is this a super tiny car? What exactly is happening to the seat? Please be specific. I’ve had 6’8" 450 pounders in my bucket seated SUV and the seats shrug it off.
Extremely thin people are a problem as well. They tend to get down in the cracks between the seat and the back, taking spare change they find and spending it on things other than food.
I don’t know about seats but I had a friend who was 480lbs in her last year and she used to ride in my little '84 Pontiac puddlejumper. It whole car would drop to her side and I was always afraid the shocks would go. I never figured out how to say no, but I don’t think the car would have lasted if it had been an everyday thing. This was just to church and back.
I’m 320 and use one side of my couch a lot, and the foam is definitely scrunched. But I’d have to ride my car a lot for many years to achieve the same effect.
ETA: My Scrabble friends got a new table and saved one of the old chairs for me, apparently because they were afraid I would “trash” the new more delicate chairs. I didn’t go back for weeks and only stayed when I did go back after they offered a chair like everyone else’s. I did manage not to collapse it. So, short answer, there’s no happy way to tell this to someone.
I’m about 320 and have been driving a Ford Contour with bucket seats for about 20 years. The driver’s seat still gives me good support. It’s still nice and springy and I haven’t broken the back on it yet.