Mini-Yoshis are a subspecies of Yoshi. Rather than having the same color skin as the spots on the egg they hatch from, all Mini-Yoshi eggs have cyan, magenta, yellow, and (unseen due to the sprite) black spots, and their skin color is determined by external factors outside the egg before they hatch. This opens up potential for them to have rare shades, hues, and color combinations not seen on normal Yoshis.
From the time the hatch, Mini-Yoshis all have the same proportions as normal adult Yoshis, but they never grow bigger than one brick block tall.