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“Petite Capitals” was translated as “mayúsculas diminutas”.
“Small Capitals” was translated as “mayúsculas pequeñas”.

However, there are established, well-known terms in Spanish to describe both styles:

Petite caps are called “miniversales” and small caps are known as “versalitas”.

Both terms can be found not just in design books at your favorite library, but are also included in Microsoft’s terminology collection.