If you have large pockets (not necessarily cargo pants size), the Poker ][ is a very nice pocket-size board. Mine has an aftermarket aluminum case, which means it's a very tight fit in my pants.
Vortex has since discontinued the Poker ][ in favor of the Pok3r, which comes with a stock aluminium case and has a different default fn-layer mapping. Other 60% keyboards should be about the same size as the Poker series.
If you're looking for something even smaller and are willing to live with using the numbers from a function layer, the JD 40 (since discontinued, or at least no longer listed on the manufacturer's website, https://www.crtralt.io) was a good bet.
Assuming you do not mind both a the 40% size and an ortholinear layout, the Planck is your way to go. Its manufacturer also makes parts for the Atomic, which is an ortholinear 60% keyboard that you get to assemble yourself.
Finally, the TypeMatrix 2030 is deeper and maybe a bit wider than the other options, but is easily the thinnest of the bunch. It's a 60% ortholinear keyboard with scissor switches (the others all use traditional mechanical switches). It won't fit into any pockets (unless you count the font pocket of a hoodie sweatshirt, but it will easily fit into a briefcase, backpack, or on top of a stack of books.