I thought of this question after seeing a post about hot shot cannon balls: https://www.reddit.com/r/history/comments/59dwyo/why_would_you_heat_up_a_canonball_before_use/
If you can fit a hot cannon ball (which obviously expands) down whatever cannon you have, does that not suggest the bore was too large for the cold/normal shots, in the first place?
What sort of sealing material (only thing I can think of) were they using to 'make up' the diameter of the standard shot, in the first place?
What type of tolerances could be expected of cannon bores?
"Sealing material":
The term you're looking for is "wadding", which is used for muzzle-loading weaponry as well as certain modern ammunition like many types of shotgun cartridges.
I'm most familiar with naval artillery, but this should generally apply for most muzzle-loading cannon.
The gun crew would load the powder charge first, either loose with a ladle, or in a cloth/parchment packet from a ready bucket. This would be rammed into a cylindrical or spherical chamber at the breech of the gun called the chamber.
Following that would be the first wad, this could be paper, cotton, canvas or some other textile, perhaps packed with old rope for bulk. This would contain and hold the powder charge in place as well as providing a base for the shot to rest on, as well as containing the propellant gases when firing to stop them escaping around the shot and consequently the loss in power.
For heated shot, the wadding would be soaked with water to prevent premature firing.
After that the shot was loaded, if it was round shot, the second wad would follow to stop the shot rolling out due to the roll of the ship.
The tolerance of the bore as compared to the shot is referred to as "windage". Different countries had varying proportions of bore to shot. A good contemporary overview can be found at this Google books link for A Military Dictionary by William Duane, 1810.
The benefits of having the bore tight around the shot were known, even with smoothbore artillery. The shot would fly straighter since there would be less deviation as it traveled down the bore, and it would fly farther since the propellant gases would be better trapped behind it.
However, the biggest down side of a really tight bore was that the barrel would expand with repeated firing - eventually making it impossible to load the gun, or worse, it would trap shot which had already been loaded, increasing the risk of a misfire or even bursting the barrel.
This was enough to prevent use of very tight bores until advances in metallurgy allowed much higher bore pressure, as well as the advent of rifled artillery and modern shell design.