Like Darryl said a cartridge being overbore is easiest to see in the very extreme cases.
A 50BMG is such a case, w/a case capacity of 290+ gr of H20 and average load being 220-240gr of powder. SSK industries now manufactures rifles chambered in 50Peacekeeper,
A BMG flanked by 2 Peackeeper rounds. A 460WBY case necked up to .510 cal. With a shorter barrel and half the powder they can achieve over 80% of the BMGs velocity with the same bullets.
A little more scaled down is my 270-300Ultra, It has a capacity of about 112gr H2O, vs a 270win w/a cap of 67gr H20. My ultra in a 30" barrel uses 103gr powder to push a bullet 3750fps while my 270win in a 28" barrel pushed the same bullet 3200fps using only 58gr of powder. The ultra takes 178% of the powder of the 270win (that's a 78%increase) to push the bullet 118% of the velocity of 270win( that's an increase of only 18%). It took 45gr extra powder to increase 550fps. That is very overbore, and very inefficient.
The most efficient cases are the smallest, a 300whisper, for example, can push a 240gr bullet around 12-1300 fps using 13-15gr of powder where a 300 Ulra pushes the same bullet 3000fps w/close to 100gr powder. The whisper has 43% of the ultras velocity while using 15% of the ultras powder. That's alot of speed for not alot of powder.