Its not possible to build a bike smaller than a certain size without compromising up the geometry. At the time the big/small wheel concept was popular the rationale was that using a 700c rear wheel allowed you to use normal cogs and chainwheels and have standard gear ratios, which you couldn't do with 2 24" wheels. IIRC 650c wheels didn't exist yet and small cogs weren't available yet, though the would be soon.

The first big/small I ever saw was built by Bill Boston. I dunno if the idea was original to him or he copied someone else. Terry was the first to market a line of bikes specifically for women and she used the big/small wheel design on her smaller sizes.

My ex-wife had a custom 700c/24" bike. It worked well for her.