I went to try a horse for loan, an 11yo warmblood, and he was in a snaffle with enormous keys. The owner said she thinks he likes to have something large in his mouth to "hold on to" - this is weird, no? Would you happily ride in a bit with keys long term? The horse did not crunch them as a youngster would. He tended to overbend and was a little resistant in downward transitions and that was even with me trying very hard to be light handed.
I am thinking of asking to get his teeth checked.