Originally posted by Teral:
What they meant was that he would start each episode at his normal weight and then slowly get fatter as the episode progressed. So he wouldn't be that much fatter than at the end of "Love's Labors Lost in Space".
True story: William Shatner had a tendency to put on weight back when the original Star Trek was being produced. He'd get fatter and fatter until the middle of the season, then go on a crash diet and be back to his original weight at the end of the season. Having Zapp get fatter throughout the episode could be a reference to that.