Keybplayer, my respects to you, but...
If I were you, I wouldn't worry about why Yam didn't reveal the amount of WAVE ROM.

I can't see how the size of ROM found on one keyboard could affect the sales of another. The keyboards you listed are of different kind - Tyros is an arranger, Motif is a workstation, BOTH made by Yamaha. What's more, you can add your voices into both keyboars.
IMO this is definitely a marketing trick, but probably the goal here is not to let you compare the ROM of Yamaha's keyboard to the rom on keyobards made by their competitors.
Anyway, size of rom is not important, what is important is how the keyb sounds.
Best regards,
George