Many gun writers don't really understand bullet expansion and penetration, partly because so many don't carefully set up and analyze the tests they make--if the make any. Which is why you see the repeated claim that higher weight retention always results in deeper penetration. This simply isn't true, and while it's easy to prove, many people (not just writers) just repeat what they've heard or read. This has been pretty good for me, because I've sold a lot of articles that actually tested many old assumptions, which either were true at one time but aren't any more, or never were true in the first place.
I believe the fixation on bullet weight retention started with Bob Hagel's book, Game Loads and Practical Ballistics for the American Hunter, published in the 1970's. Before Hagel there wasn't much real testing of big game bullets, but there were only two real premium bullets back then, the Partition and Bitterroot Bonded Core. Hagel emphasized weight retention constantly throughout the book, but also mentioned at least once that the BBC didn't penetrate as deeply, because of its wider mushroom. But most readers (including a lot of gun writers) mostly remembered the emphasis on weight retention, which is why so many articles, books and hunter gossip emphasized weight retention as the Great God of Bullet Performance.
The other reason I suspect weight retention got such pay is it's much more easily quantified than wide expansion, so is more easily bragged about when hunters keep score. And most humans keep score in some way, whether or not the score means anything.
The most extreme field example of wide expansion limiting penetration I've seen was a 360-grain bonded bullet from a .416 Remington Magnum that didn't exit during an angling shot on a 100-pound deer. The bullet retained around 90% of its weight, but also expanded to over 2-1/2 times its original diameter, with only a very short shank left unexpanded. If it had been an actual mushroom, it would have looked like the mushroom's top was sitting directly on the ground. There's no way a bullet that wide will penetrate as deeply as a bullet that expands into a long-shanked mushroom with a small "top."
Another common and mistaken assumption (that you also alluded to) is that different expanding bullets expand faster or slower than others. Instead, the vast majority expand completely by the time they penetrate their own length. Instead, the difference is in how much they fragment, not how fast they expand.
The exception to this, oddly enough, is very long-ogived "target" spitzers with tiny hollow-points. The needle-point tends to delay expansion until the bullet penetrates 2-3 inches, whereupon expansion is usually violent because of the thin jackets common to such bullets.
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