Looking at the Wikipedia page for the J-2 (one engine was used on the Saturn V third stage, and 5 engines on the second stage) it gets about 232Klbs thrust in vacuum, but at sea level, only 109KLbs thrust.
That is a pretty wild performance swing.
Why such a difference? If you compare the Vacuum Merlin's vs the sea level Merlin's performance you get a much smaller swing in performance.
I am guessing it is because there never was a sea level optimized J-2 engine (it was meant as an upper stage engine) so the sea level number may be spurious or only from test stands.