The folded leather billfold that once shaped the back pocket of American trousers has been replaced by something closer to a card holder. Several separate changes produced that result.
Cash volume fell first
A traditional billfold was sized around a stack of banknotes, which is why the note compartment runs the full width of the folded object.
As card and phone payments took over routine transactions, most people stopped carrying a working cash float. The compartment that defined the wallet's dimensions emptied.
Cash use has not disappeared, particularly for tipping and small vendors, but the amount carried is smaller. A slim wallet still holds a few folded notes.
Cards moved into the phone
Payment cards, transit passes, loyalty cards, boarding passes and increasingly identification exist in digital form on a device already carried everywhere.
Each migration removes a physical card from the stack, and the stack is what sets a wallet's thickness. Removing several cards changes the object's category.
What remains is typically a small number of cards that either lack a digital equivalent or are needed when a phone battery fails.
Receipts and paper clutter went to email
A significant portion of an old wallet's bulk was accumulated paper: receipts, appointment cards, tickets and notes never removed.
Digital receipts and calendar reminders absorbed almost all of that. The wallet stopped functioning as a filing system.
That change is easy to overlook because it was gradual, but it removed more volume from many wallets than the cards did.
Thinness became a design constraint of its own
Once the contents shrank, wallet makers competed on thickness, and materials followed. Elastic bands, machined metal and single-layer leather replaced multi-layer stitched construction.
Those constructions carry trade-offs. Metal holders can abrade card surfaces, and single-layer leather offers less protection against bending.
Capacity limits are also real. A holder built for a handful of cards becomes unusable at a dozen, which is why people cycle through several designs.
The pocket was always the constraint
A back-pocket wallet has to survive being sat on, which flexes and eventually cracks whatever it is made from. That is the dominant wear mechanism.
Thinner wallets suit front-pocket carry, which removes the flexing entirely and extends the life of the wallet and the trousers alike.
The shift is therefore not only about contents. A smaller object made a different carrying position practical, and that position is easier on everything involved.