The term ATX does not refer to the physical size of power supplies. The physical dimensions of ATX power supplies can vary. Here are examples of two popular power supplies:
5.88"(W) x 3.38"(H) x 6.50"(L)
5.9"(W) x 3.4"(H) X 5.9"(L)
It's usually the length or depth (distance from front to back) that has the greatest variation. On the other hand the screw holes for mounting an ATX psu will match the psu screw holes in ATX cases.