The OED, 1st print ed., gives the following for bouncing: "That bounces; in various senses of the verb relating alike to loudness, brag, and vigorous or ungainly movement. Often also (like thumping, whacking, whopping, strapping, and other words meaning vigorous, striking) used with the sense of big rather than elegant or graceful." A couple of few citations use the adjective with some wenches. a priest, and a puppy. Here's: "His mother ... lay down her burthen at Elmeby, where this bouncing babe Bonner was born." 1662. Thomas Jefferson is quoted in re a bouncing letter. I'll admit that "bouncing baby (boy)" has a nice alliterative effect to it, too. But I think it was applied to babies at a time in the past when healthy a robust indicated a better chance of survival. There was also animated short about a baby Gerald McBoing-Boing in the early '50s for the rubber baby buggy bumper kinda bounce.
