Two-Bits,
Generally when lighter bullets are used there is more surplus airspace or "case capacity" in the cartridge compared to the same case loaded with a heavier slug. There are exceptions, the best known being the Barnes X-bullet. Those excellent, all-copper projectiles are rather long for their weight compared to conventional bullets with lead cores. So they take up slightly more case capacity than a same-weight bullet with a lead core. Bullet weight and length are the two main reasons why you can put more powder into a .30-06 loaded with 150's than you can the same cartridge loaded with 180's, for example.