I'm not fat per se but I'm curvy and some things are hard to get over my boobs or butt. And my mom is a little bigger. Why do we pay more? Is this society or because of a little extra fabric? Are we bad people? Is it hard to make?
Partly it is the fabric.
Partly it is that they have to partially re-design the pattern for the larger size.
Partly it is because they make fewer pieces of an item in a larger size .. and in any manufacturing system, the MORE pieces you make, the more profit your make . and the fewer pieces you make the less profit you make per piece.
Mostly it is because they know you have no alternative.