Zelda suggests she and Mr. Kraft name all of Jupiter's moons by mass, however, the order they go in has nothing to do with the mass of the moons they name. Zelda starts with Europa which is neither the smallest nor largest and Mr. Kraft ends with Callisto which is smaller than his previous answer of Ganymede (which is the largest). Since Mr. Kraft also says Metis we can assume they were using all the named moons and not the four largest.
Mr. Kraft adds an R to Jupiter's moon Metis and mispronounces it as "metris."