The argument the article makes is that the pie is getting better-tasting. That is, because Bob can derive more enjoyment from a fixed volume of the new pie, it

The argument the article makes is that the pie is getting better-tasting. That is, because Bob can derive more enjoyment from a fixed volume of the new pie, it doesn't matter that Bob now takes home a quarter-slice, instead of a third-slice, so long as the net volume of his new quarter is at least as large as his old third. It argues that Bob is still being rewarded fairly for his continually increasing productivity, even if his net volume is stagnant and his share of the whole pie is falling, simply because the pie is getting better tasting.