The leaves on my mandrin tree are different sizes. New leaves are very small and somewhat curly but no blotches. The older leaves are dark and shiny.
Mandarin Orange Leaf Size
Certified GKH Gardening Expert
Insects or disease can cause malformations in leave structures, as well as environmental conditions. Temperature fluctuations and watering practices.
You can treat the tree with Neem Oil, as this works as both an insecticide and fungicide and is safe for people, pets and bees.
Here are some links for you.