I have been reading alot on this topic and from what I gather Bullet fit is important, because gas leakage around the bullet will cause it to melt the bullet and produce a lead spray. If you don’t have a good fit, then all other attempts to minimize leading are really a waste of time. I am not an expert but I have read this in alot of articles. If you meant maybe your bullets are smaller than your bore this could happen and you still need to have the things hammer talked about to reduce it also.
No - I do not believe that there is any relationship to size bore -vs- leading. I shot cast 9mm all the way up to 500. The things I look for when leading is present - in this order:
1. Size and fit of boolit to bore. The diameter of your cast boolit should be .001-.003 larger than you bore diameter. The only way to know this for sure is to slug the barrel.
2. Condition of bore ... bores with pitting will lead a lot faster than a good sharp bore - the pits seems to want to fill with lead and pull lead from the boolit.
3. Velocity of boolit - for non-gaschecked boolits try to keep your velocity around or under 1000fps - I pushed some HARD .223 gaschecked 55gr boolits at 3012fps the other day without leading. For most all handgun loads velocity should not be much of a problem.
4. Alloy - try harder first! if that does not work, try some softer alloy ....
5. Lube -- many different kinds out there and many different lube grove styles on boolits. Try a different lube.
6. Boolit Style - There are a few bores out there that just do not like a particular style cast boolit - try a different cast boolit style.