Bullet size is a factor but so is bullet design. The original bullet was only .266, but was designed to flare a couple thou at the base when shot to reduce gas cutting.
The Hornaday bullet is .267 it's whole length, but with a hard solid base, which is a different approach to making a bullet than the Italians used.
Privi makes a .268 spire point that is popular for these, but it's got a short contact surface to avoid overpressure.
Then there are the .264 bullets that shoot ok in some guns, but shoot poorly in most. Largely this is because people are shooting boat tails or solid base bullets that aren't obturating and are suffering from a lot of gas cutting. Plenty of a .264 bullet still grips the rifling, but with a less than perfect crown, gas cutting can make a bullet tumble or wobble as it leaves the muzzle, reducing accuracy.
Some other bullets, I think the norma bullet did this back in the day, had a .268 driving band near the base of the bullet that was otherwise smaller in diameter, .264 or less.
Anything over .256 will grip some of the rifling.
More than one way to skin a cat, I guess.