There's many things I blame showing on but the current size of TWH isn't one of them, that's the fault of baby boomers! Boomers are wanting smaller horses to climb up on and they are rather large force in todays market. I've talked to quite a few breeders that say they have a heck of a time selling anything over 15.2 and forget selling ones over 16. So they breed for shorter walkers instead of the monsters.
At 47 years old I prefer the shorties myself. Back in the day I only wanted to ride the tall ones, but now I splat instead of bounce Less distance to fall from a short horse