Why do the phasers in the original Star disintegrate everything but not in later series?

In the original series, whenever someone used a phaser to kill someone, they ended up disintegrating them but in the newer series, whoever uses it, the phaser just kills the person but they're still there. Against the Borg, for example, I imagine in the original series they would be vaporized but they shoot sparks when shot by the phasers in the Next Generation.