I have some experience with some big stuff off the bench, .375 Ruger, .375 H&H, .416 Rigby, .458 Win mag, .450 Nitro Express etc.
The biggest factor in groups opening up is body fatigue 90% of the time.
Even if you're a big guy (I'm not, 5'9" and about 160lbs) the pounding you get from bench shooting takes its toll.
Standing or sitting is a whole different ball game, my kids love shooting my .375 Ruger, but they shoot it standing.
My oldest son put twenty rounds through it in about half an hour the last time I had it out.
He rang the gong every time at about 50 yards offhand with hot 270 grain loads at about 2850 fps, he wanted to keep going but I was out of ammo.
You can soak up more recoil COMFORTABLY if you don't use a bench, that crazy kid is about 5' and maybe 110lbs.