I'll try.............
.... but there are many more learned folks that may comment further.
My concept of 'bore' capacity, or the 'over-bore' condition, is when a case is enlarged to hold bigger powder charges, and the velocities gained from the extra powder do not reflect the powder increases. i.e.: In an ideal situation, you should see significant velocity increases (relative, and subject to personal interpretation) for powder increases. When the velocities don't increase much, or at all, you have reached the 'over-bore' condition. This is assuming you stay within 'safe' pressure limits.
At that point, you are burning excess powder for the bullet, barrel length, etc, and providing undue heat and erosion in the throat, contributing to needless barrel wear.
Someone please add to my knowledge.
-West