The MacBook Neo has only 8GiB of RAM. This is widely decried as not enough. The RAMpocalypse means that 8 GiB is going to have to be enough or else it'll eat into Apple's margins. The Neos are wildly popular though because people have used it and 8 GiB is fine. So while the singular weather app being smaller itself isn't material, in aggregate, macOS being more efficient is something that lets Apple sell a lower spec product while still justifying a higher price and thus better margins for Apple.