No arms race lasts forever. As weapons get bigger they also get dramatically more expensive. Eventually, populations reach a new balance where the now-higher costs neutralize the reproductive benefits. Bigger stops being better, and the arms race stalls. Populations stabilize, hovering at the new weapon size. How big the weapons are at this stalling point depends on where the balance is finally reached; animal weapons under strong sexual selection, for example, may attain astonishing proportions before costs catch up and place the process in check.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 432