When Tyros only came in 61 keys everybody was asking for a 76 note version, when the T5 appeared the 76 outsold the 61, so it was logical to make the Genos a 76 as it is what most users wanted. As to weight, then the Genos is lighter than a 61 Tyros and not much bigger, so you have the best of both worlds.

There was never any reason for an arranger to be lumbered with a highly restrictive 61 notes, (It was just a throwback to the organ, but they forgot that an organ had 2 of them) particularly as a lot of them are used as pianos. (Have you ever seen a 61 note acoustic piano?)

Bill