Snow's Rubyshroom
Snow Island arose from the sea as a barren island, to be colonized by various species. One of these species was the Rubyshroom. It arrived with other crystal flora, all of them being very large species, towering over it and blocking out the light. This was much like the environment it was adapted for, a life in the undergrowth using the light the larger flora wasn't using.
The path to grow larger was blocked off by its larger neighbors, but the path to grow smaller was open. Snow had no herbivores, so smaller Rubyshrooms didn't risk getting eaten, and thus the main factor to change them was available space. As time went on, the smaller Rubyshrooms would have more and more members and the population transitioned to smaller and smaller adult sizes until it finally reached near a fifth of its ancestor's size, growing to only 6 cm tall. By this point, it was a new species and had out-competed its ancestor in its range. Snow's Rubyshroom had out-competed its ancestor simply by taking control of the same space and doing so quickly enough to beat them out.
Snow's Rubyshroom reproduction has increased in effectiveness. The airborne spores are released after rains, and the waterborne spores are released almost constantly. Budding occurs much more frequently and operates much as runners do. A single Snow's Rubyshroom can take control of a square meter of forest floor through its runner network, effectively choking out any competition in the area from germinating there. The larger crystal flora are thus selected for more durable spores and juveniles.