Slightly smaller. Look to JPN's website for sizing information.

AFAIK, most photo shops just treat them as them as the same size. Back in those days there were only two photo sizes: passport and license.

However, JPN have been taking their own photos digitally for many years now. So "IC photos" don't exist anymore. These days jpn do not want you to bring printed photos. I doubt they will list the size on their website, unless it's some old page they have overlooked and left up by mistake.