Several factors, first off the 375 H&H is typically perceived as more stout shove than the 300 wm which is "snappier". Slower moving bullets tend to have this characteristic vs. faster magnums.
My 505 Gibbs has more barrel rise and shove, but feels like less "kick" shooting a 600 grain bullet at 1900fps than a 525 grain at 2350. It actually hits 8" higher with the heavier and slower load. I'm told that is because I'm getting more barrel rise before the bullet exits the barrel... Sort of makes sense. Ballistics and rifle reaction are very interesting
And perception can mean a lot. After shooting a 505, a 375 does not kick much.
Many common 375 rifles will be a couple pounds heavier than most 300's. That makes a big difference, as well as stock design. Winchester M70 Express and CZ 550 are different but both designs help control recoil. And a 375 will typically have a heavier barrel profile, helping to hold them down. For example, a lot of 375 and 416 barrels may have the same outside diameter, thus less metal in the 416 as the hole is bigger. So that can compound the felt recoil of the all ready heavier hitting 416.
There some lightweight 375's that will kick the hell out of you. Such as the synthetic stocked Browning A bolt.
Finally with any big bore, you want to be holding on tight! Pull it into your shoulder pocket and get your cheek down tight to the stock. Have your scope mounted low enough. If you don't do this, they will slap you around. Hold her tight and she will slow dance with you
