That has little to do with technological advances, just the fact that the US is at an imperial peace; ie. it is under zero threat of invasion and is currently only engaged in small-scale imperial adventurism across the globe which does not require a large standing army. ~16% of adult Americans served in WW2, or ~33% of adult men.

Is Iran enjoying an imperial peace, too?

Even including Aative + reserves + paramilitaries, less than 2% of Iranian adults are in the military.