Mario to Bowser in Smash 4 is 3-4 times smaller than Mario to Bowser in Super Mario Galaxy.
By comparison if you took Mario size next to Samus's size in Smash 4.
(Which, fyi, Samus and Bowser in this game are almost the same height here,)
Ridley's size in Other M wouldn't be any much more bigger than Bowser in Galaxy, if not a bit smaller.
Ridley is roughly the same size if not a bit bigger in Smash by comparision to OM Ridley. Bowser in Galaxy should be the same size, and this is when he's usually his average size in the Mario games. Most of the other times, he's much bigger than Ridley ever can be, even to his Meta Ridley form (and no I don't mean what's in Smash 4, that doesn't exist in Other M)
This is canonically Ridley's biggest incarnation, about 1.5x bigger than his normal form which would make him still like 4-5 times bigger than Samus, to normal sized Ridley being like at minimium 2-3, 3-4 times bigger at best.
Which even than, that's still smaller than Bowser's biggest forms,