names - Why does Rashi call tzeirei and segol "kamatz katan" and "patach katan"

Rashi often refers to tzeirei as "kamatz katan" and segol as "patach katan". Why? In what sense are they smaller versions of kamatz and patach?

I understand that kamatz and tzeirei are תנועות גדולות, while patach and segol are תנועות קטנות, which has some consequences for sheva na and dageshim. Rashi's classification is going in a different direction. Does it also have grammatical consequences?

             Tenuah gedola       Tenuah ketana
Rashi big    Kamatz              Patach
Rashi small  Tzeirei             Segol