As long ago as the late 1700s, a 'slapper' was anything large or overgrown. It later came to be applied especially to women fitting that description. Over time, it came to refer to a woman for almost anything pejorative about her...size, behaviour, morals or whatever.
One of the earliest references, Jno, was to a large salmon...in much the same way the earliest references to 'flappers' were to birds (feathery ones). Easy enough to see, I think, why such a thing as a salmon would be called a 'slapper'...ie it flops and wallops around after being caught. Similarly, presumably, large ladies were considered to be such as 'flopped' and 'walloped' around as well!