How so? There were very few episodes of TNG in which each character played a vital part. In most of the Riker-centric episodes, for example, you wouldn't need someone with a Visor.
But with TOS and methods of its day, the focus has to be on Kirk, Spock, or maybe McCoy, because they get the biggest paycheck. Then you divide more lines between Scott, Sulu, Uhura, Chekov, and maybe a Rand or Chapel, and one dialogue extra. And you're recutting from 5 acts to 4. Also, longer running time so you need more scenes.