What's your dataset here? I don't think this is even slightly true, looking back through WoW's history, they've been all over the place on this - some expansions have multiple significant patches in terms of world content/zones/etc, others have none that really stand out.
Sure we can go back to each expansion, and retroactively say each expansion had one patch which added the most stuff. Obviously. Just like we could go back through your TV viewing habits in a year, and find that each year you watched one show more than others. That doesn't mean you had "one big show" each year.
So unless you have some compelling data here, more compelling than the obvious fact that, technically, each expansion has a biggest patch (...), I'm going to have to say this seems like a complete asspull of a claim.
I mean just looking at Legion, Broken Shore seemed pretty big until Argus came out. I wouldn't be shocked to find 8.3 to be a similar size to 8.2.