Although similar in appearance to bulborbs, these belong to a totally different species. It not explained why.
Dwarf Red Bulborbs are small and very common enemies in the Pikmin series. In Pikmin 1 they were known simply as Dwarf Bulborbs, until various subspecies were discovered in Pikmin 2. They are revealed to be in a whole different family of species than the regular Red Bulborbs at the end of Pikmin 1, and are later found to be in the Breadbug Family, where as the normal Red Bulborbs are in the Grub-dog Family.
Dwarf Red Bulborbs (アカコチャッピー) are small and one of the most common enemies in the Pikmin games. They were originally known as Dwarf Bulborbs in Pikmin until other various subspecies with different coloration were discovered. At the end of Pikmin it is known that they are not of the same species as the Spotty Bulborb, and in Pikmin 2, they are revealed to belong to the Breadbug family. In Pikmin 3, they are referred to as Dwarf Bulborb once again, despite there being more types of Dwarf Bulborbs in the game.