In Gironda's time, the winning physiques were based around having a significant V taper between the shoulders and the waist. He reckoned the easiest way to do that was to just keep the waist slim. If you look at pictures of Gironda, you'll see that he was not a particularly big guy himself, but he was a master of creating certain lines in silhouette.
Also keep in mind that he spent a significant part of his career training actors to look in shape when on camera. He was heavily focused on creating the "illusion" of size, rather than actually creating size. You can't really add much size to an actor in just a couple weeks, but you can make them appear bigger on camera. This was his specialty, and it's important to understand what he says in that context.
Just a few decades after Gironda, bodybuilding put a lot more emphasis on actual size, not just shape. The person with the most muscle, and the best conditioning, wins. Shape still mattered, but a skinny guy with great lines (e.g., Gironda) would lose out to a larger more muscular guy (e.g., Ronnie Coleman), or without very good lines at all (e.g., Branch Warren).
In general, it's a better approach to work on making your weak points larger, than to try and keep your strong points from getting "too big".