In the case of our situation, it's more yield per seed, so doesn't vastly increase the labor: you're still harvesting the same number of plants, they just produce bigger grains etc, or more grains per plant. Threshing might take a little longer, but it isn't the same increase of labor as bringing more land under cultivation, which historically has been one of the major ways to boost production (since then you have to prep the land, do more planting, do more weeding, do more harvesting, etc).

Harvesting magic can be our next infrastructure improvement. :-)