Pym Particles Ant-Man is able to reduce himself to the size of an ant, approximately one-half inch in height, by means of a rare group of sub-atomic particles, the source of which is as yet unknown, which he is able to contain in magnetic-field "canisters" (the shape of the magnetic lines of force were made visible by the confined particles, whose concentration caused them to behave like a gas).

These particles, whose wavelength is in the mental range when released, allow Ant-Man to will himself and other objects to reduce in size or enlarge. The mechanics involved having most of the mass of the nucleus of each affected atom convert into energetic particles which take up orbit around the nucleus - thus, although gravitons do not affect these particles, the mass of the original volume remains intact.
This means a punch delivered by the half-inch high Ant-Man would feel like the blow of a normal-sized man. Ant-Man's reducing particle has no time limit to its potency.