It should be said using near throat-size bullets is principally for best lead bullet accuracy (bullets equal to throat diameter or half a thousandth under throat diameter seem to be about ideal). Jacketed bullets are less affected. You can prove the effect on lead to yourself by checking your throat diameters with pin gauges. It is not uncommon to discover they are all different. Production methods seem to have a different reamer finishing each chamber. And if you pick the chamber closest to the size of your lead bullet without being smaller, and then load and shoot a group entirely from that same chamber, it will typically be tighter than groups fired either from the other chambers or from a mix of all the chambers. Lots of experiments have shown bullet base erosion by propellant gases bypassing the bullet while it is in a loose throat deteriorates accuracy. It builds up forcing cone and throat lead deposits as well as lead in the throats themselves.
Squeezing bullets (sizing them) in a gun's throat typically produces a pressure rise, but it isn't huge. Even oversized jacketed bullets (wrong caliber) have been fired through high power rifles at times and usually without apparent damage, though this does raise pressure and should not be done on purpose, and accuracy is poor. Lead bullets, if you've ever sized one, do not demand particularly high pressure to get them narrowed, and thus are not a hazard to shoot this way. Still, ideally, you have throats one or two thousandths over groove and bullets to match, as minimizing bullet distortion also helps with accuracy.