When was the last time you measured your waist and then tried on a pair of pants?
Men’s clothing has tones of vanity sizing these days.
The cuts of fabric are cut with dies. A layer of fabric is place down and a press presses the cutting die down to cut out the shape. A cheap manufacturer over stacks how many sheets are cut at ones. Top layer is going to be bigger then the bottom layer.
I expect there is much more hand cutting going on than you realize. To have multiple styles and different cuts would require giant warehouses of dies. Those aren’t cheap and wouldn’t last long in a production environment. Any change would require a new die. The machine shop would need to be as big as the cut sew shop.
One skilled (or trained) operator can change from pants to shirts on a whim. You’ll notice almost all clothing is made in far off places where labour is cheap. Not to say there isn’t die cut stuff, but overwhelmingly the textile industry is hand made.
These things are how multiple layers of fabric gets cut.
Surprisingly CNC cutting machines are becoming more common even in places where labor is “cheap”. The technology is becoming affordable and reliable enough to replace even underpaid workers as a “cost saving” measure.
Here’s the garment cutting department at a factory in Pakistan.
For pants? At scale, the dies are cheap and probably can be reused across multiple styles within the same brand and size. The difference between blue jeans and cargo pants isn’t the cut. It’s the fabric and accessories.