The smallest by volume I can think of available at Amazon is probably the Silverstone Fara R1, at 36L in volume.

The next smallest is probably the Fractal Meshify C (metal panel version and glass panel version) at 37.9L. Neither are sold or shipped by Amazon though, so don't know if that matters.

The Darkflash DLC29 is a fraction bigger at 38.5L in volume.

The Vetroo AL-Mesh-7C is getting into more regular ATX volume at 40.9L, but is still relatively compact while able to fit pretty large components.

For reference, the Thermaltake Core V21 is 45.5L in volume, while the NZXT H700i is a little over 58L.

If you want an ATX case that is smaller while still being able to fit standard components (and ATX power supplies), you're going to have to purchase from other retailers.