Nuclear weapons development hasn't stopped outside of US. Maybe you mean it hasn't spread to any new countries lately... which is true but I wouldn't count on that lasting.

Both USA and Russia, have decided that ~Megaton Hydrogen Bombs are the biggest we're going to get and we don't plan to build anything bigger than that ever again.

The USSR wanted to make sure they were the ones who built the largest bomb of all time (the Tsar Bomba at 50 MTon). And after that, development on more ferocious weapons has stopped.