Case heads and case rims can expand when cases are defective or over-pressured. I've measured case head expansion multiple times in the past 15 years. But in every instance, the primer pockets have also expanded. When primer pockets expand, the case web and case head also expand.
If the primer pockets have not expanded to a larger diameter than on unfired virgin cases, then it's very unlikely excessive pressure is the primary cause of the case heads not fitting inside your shell holder. Case rims often aren't exactly 100% round. Sometimes I've rotated the case heads slightly and they will then enter into the shell holder. I've also mounted cases in my drill press and slightly reduced the diameter of the case head, with a fine toothed file and some sand paper, until they slip into the shell holder.
You can buy pin gages on FleaBay and Amazon to check your primer pocket diameter and test for loosened primer pockets (expanded primer pockets). They aren't expensive. I use them to track both small and large primer pocket diameters of my casings.
I'd try another shell holder, or two. You might find your's is simply on the tight end of manufacturer's tolerance. Also consider buying a different brand of brass. I've had some inconsistencies with Hornady brass, in years past. Especially their 375 Ruger cases.