For an example of what I mean - in the first campaign mission you start on protoss I, which is a quality 12 planet, and have protoss iii, which is a quality 8 planet right next to it. By default they have 12 and 8 tiles respectively.
If you take magical planet quality as a trait, protoss I will be quality 14 when you start and have 14 tiles. Once you settle protoss III, however, it will go to quality 9, but will still only have 8 tiles.