Re: Why "Jenny"?
Bob Hutchins in central TX
>As WoodburnBob posted above, it's to do with mules; or more specifically with asses (donkeys). A stud donkey is called a jack. Jacks are bred to horse mares (pony mares, too) to produce mules. Mare asses are called jennies (jenny in the singular). Sometimes, horse/pony studs are bred to jennies and the resulting offspring are hinnies (hinny in the singular).
It's not too big a linguistic leap for old-time carpenters (before WWII), who were quite familiar with mules and hinnies, to name a plane smaller than a jack, a jenny. And you're right, jennies tend to be a bit smaller, but not necessarily rounder in the rump.
BobH, who used to ride mules as a kid and who has known a few jack asses, too.