The issue isn't just the 18 lb, it's that one tiny paw that they're putting all of that weight on, that just so happens to be resting on a rib, or worse between 2.
Some cats are just big, my dude. I've got an 8-month old Ragamuffin who's already 13 lbs / 6 kg. He's expected to grow past 20, it's normal for his breed.