no airline is operating a 624 seat all economy 747-400 - and no airline is operating a 550 seat 777-300 either. But keep in mind the 747-400 Domestic JL and NH use (568 seats) are two-class: there are 24 (!) F seats on board - the number of seats in theory in an all economy configuration could be 624.
Now, where did I get this number - hmm, good question. I guess I didn´t bother remembering the source when I first heard it, sorry.
The thing is, when comparing planes´ capacities, I like to calculate with the highest possible number. Because otherwise, you get "standard three class configurations", and then you don´t know what exactly is meant by that.