Yoda and Miss Piggy are totally different characters and one would never suspect that the voice actor for both is the same person. After all, one's male and one's female.
Frank Oz not only did the voicing, but also most of the puppeteering work and is responsible for the character's quirks. This includes the 'Yodic' (object, subject, verb) style of speech used by Yoda in the Star Wars movies.
Oz has also done the voices for the Cookie Monster, Fozzie Bear, Animal, Grover, Bert (from Bert and Ernie), and Sam the Eagle.
He directed The Muppets Take Manhattan, What About Bob?, The Little Shop of Horrors, The Indian in the Cupboard, The Stepford Wives, The Score, and number more famous films. Oz also co-directed The Dark Crystal while voicing Aughra (the oracle creature).