There are many types of Tieflings, but they share certain Tiefling traits as a result of their infernal heritage. The DnD stats above belong to a specific kind of Tiefling, known as the ‘Asmodeus ...

These rule that a Gnome would be too small to fit in the armor of a tall DnD Tiefling, for example. The Player’s Handbook says you can only mount “a willing creature that is at least one size larger ...