6% on the brass I've measured for h2O capacity, which using the John Barness 4:1 rule, you'll see velocity increase 1% for every 4% more case size (loaded to equal PSI) - so that makes the difference about 1.5% more velocity (approx 40 fps)
Quickload predictions for each when loaded to SAAMI length OAL @ 62,000 PSI with a 300gr Nosler Accubond, average velocity of the top six performing powders for velocity :
24" 375 HH ~ 2567 fps
20" 375 Ruger ~ 2525 fps
24" 375 Ruger ~ 2625 fps
375 Ruger does have more bore travel than the H&H due to the shorter cartridge length, so that helps some small gains
Set "rules" like that don't exactly work. It has no account for the case design. The old slow tapered case and shoulder on the H&H compared to a fairly straight walled case with a 30° shoulder changes the powder burn characteristics, efficiency of burn and basically every aspect of internal ballistics. Which account to more than a simple 1% generic 4:1 rule. Comparing the same case design to the same case design, or scaling up a case, it holds more true. But not when you are comparing two things so vastly different in design.
The 375Ruger gets quite a bit more velocity than the H&H across all grain weights, and does it with a 3-4"~ shorter barrel. When comparing the two with the same length barrels, the difference is quite a bit.
I can easily get way more than 2525fps with a 300 grain bullet with my 23" 375 Ruger, with a range of different powders. I run the 350's at 2500fps with a 23" barrel. And the 375 Ruger only has .26" more barrel travel when comparing over all length, and the same length barrels.
Buy a 375 Ruger, and buy a 375H&H, and load a variety of bullets from the lightest to the heaviest with a bunch of different powders. Developing loads for each, and shooting hundred of rounds downrange, comparing recoil, velocity, accuracy potential, and quite literally everything between the two. Then you will realize there is a difference, and the 375 Ruger beats it on all accounts. Including how easy it is to load for and how inherently accurate it is. Just stating things others have said, or write about, is a whole different thing that doing the things yourself.
If you do have both, with how you are talking about them, I believe you do not, then that's your experience and I would like to hear more about it. But just stating irrelevant things and not knowing first hand, and arguing with people with the actual cartridges and their experiences with them is completely useless, as it holds no value.