Penetration is the issue. Controlled expansion bullets better harness the power of standard cartridges for good penetration and with the faster magnum cartridges, they prevent premature bullet blow-up.

Sectional density (SD) is the relationship of the bullet’s weight to its length. "Long-for-their-weight" bullets penetrate better and that’s critical on a big, tough animal such as an elk. I like a SD of at least .250 for elk and other large game.

Bullet makers tend to make their heavier bullets in a given caliber a bit tougher in the expectation that they will be used on bigger animals. Lighter bullets may have more velocity but you sacrifice both SD and bullet strength.