basic rule of statistic that the media in unable to understand, correlation does not equal cauality. In this case I suspect the men married to "skinny" women (not sure when size 12 became skinny) are more attractive themselves, thus more able to find a willing partner to have an affair with.
Totally agree with you regarding caorrelation not equalling causation......however I take exception to you implication about a size 12 not being skinny.
Being 5'10 and 115 pounds kind of falls into the skinny category.....but I wear a size 12.
I have seen some very ugly skinny women as well as some astonishingly attractive slightly overweight women.