I preface this summary by saying that this is my interpretation of the history of the show pig industry over the last 35 years.
I think for some of us that have been in this industry for several decades and have seen the type changes over the years, we could say that many of the type changes over the last 35 years or so have been done by a handful of large breeders creating the next generation of valued genetics.
I believe the show pig industry decoupled from the commercial industry in the 1980's. This is when the industry started promoting the short sided, deep bodied, fat hogs with little muscle. These hogs couldn't even meet the minimum certifications for body length, leanness and LEA. Obviously, this was not the commercial industry pushing this type of hog.
Vertical integration of the commercial industry started in this time frame and the independent swine producer either aligned with an integrator, tried to stick it out on their own, or transitioned to the purebred/show pig industry. In 1980, commercial swine prices went in the tank. Many independent breeders went under, and many signed up with packing companies to get a little better price. (that was the start of vertical integration)
With no need to stay tied to the commercial industry, some large producers moved the show/purebred pig industry to these short, fat and light muscled toads that grew really well.
Personally, my dad would not let us own a pig from 1981 until 1987 because he didn't believe in that type of hog. Even though I was born in 1972, I didn't show my 1st hog in a 4-H/FFA show until 1987. So my show career was short and sweet. It wasn't until 1990 that the hogs started to reach a type that consistently met the minimum carcass certifications.
So in the early 1990's, we got out of the toad style and went to the totally opposite end of the spectrum and produced tall, long, super lean, tight gutted, slow growing hogs. We added the pietrain to the mix in the early 2000's and made them super heavy muscled and stress positive. Since then, we have moved back the other way to a small framed hog. Now we look to be in a transition to a bigger framed hog.
Today's show pig is nowhere near the same type of hog that is produced in commercial operations.
In a nutshell, the show pig industry chases an ideal type of hog that is based on a subjective visual evaluation that can be and is manipulated at any given time.
Here is a link to some history that may be of some help.
http://nationalswine.com/about/history.php
It only covers 4 of the breeds, but it is a good start.
Also, here is a link to some photos and descriptions of boars at SGI from the early 80's and up.
http://www.swinegenetics.com/frozen_semen.php
When I look at these photos, I can't believe we thought those hogs were good!