It's more than just size
Clementines are, in a word, cute, and upon first glance, you might think that clementines are just tiny oranges. But the difference between clementines and oranges is more significant than size and adorableness. Yes, clementines are smaller than oranges, but they're also sweeter with a thinner skin that's generally easier to peel. Clementines are less acidic than your grocery store-variety oranges, as well. The reason for these differences between oranges and clementines is simple. Clementine and oranges are actually two different varieties of citrus fruits. Clementines are more closely related to mandarin oranges, also known as tangerines, which are known "to be relatively small and flat, with a reddish, easily peeled rind," explains Harold McGee in On Food and Cooking: The Science And Lore Of The Kitchen.
Though mandarins oranges were first cultivated over 3,000 years ago in India and China, clementines are a relatively new variety of citrus fruit. They were developed in 1902 by a French missionary living in Algeria named Father Clément Rodier. And though clementines are derived from mandarin oranges, there are differences between clementines and mandarins. Clementines tend to be rounder in shape than traditional mandarin oranges or tangerines, and the skin of clementines are more oily.
Despite these differences, however, the best substitute for clementines is still mandarin oranges or tangerines, not navel oranges. Nigella Lawson recommends that you make a one-to-one substitute for fresh mandarin oranges for fresh clementines, and cook the fruit in the same way—though she does note that mandarins tend to have seeds while clementines don't. If you can't find either fresh clementines or fresh tangerines, maybe because they're of season, the experts at MyRecipes recommend that you "substitute 2 (29-ounce) cans of mandarin oranges in light syrup, drained" for every 10 to 12 clementines.
But really, the best way to enjoy a clementine and everything that makes it different from an orange or a tangerine or even a mandarin orange is to eat it fresh, straight out of the peel.