Io, with a mass of 893.2 x 10^20 kg orbits in a 2:1 resonance with Europa (670.9 x 10^20 kg) and in 4:1 resonance with Ganymede (1070.4 x 10^20 kg), in orbits separated by only a few Jovian radii. They are massive, close, and in resonance. This means that Io is flexed in a periodic fashion, by strong gravitational forces. (
Jovian Satellite Fact Sheet).
Tethys, with a mass of 6.27 x 10^20 kg (0.7% of Io's mass), orbits in a 1:2 resonance with tiny Mimas (0.375 x 10^20 kg, 6% of the mass of Tethys). They are both closer to Saturn than is Io to Jupiter. Then again, not only is the mass of Saturn only 30% of the mass of Jupiter, but the close moons of Saturn are much less massive than the close moons of Jupiter. Saturn has only one satellite that can compete for size with the Galilean satellites of Jupiter, and that's Titan (1345.5 x 10^20 kg), which, while more massive than any of the Galilean satellites (barely), orbits at 20 Saturn radii. (
Saturnian Satellite Fact Sheet).
Io is in a unique tidal vice that is not duplicated anywhere else in the solar system. It is subject to more severe tidal heating than any other object that we know of in the solar system, and that's why it is the only actively volcanic moon in the solar system. (
Io: Jupiter's Volcanic Moon).
And I don't doubt that the
Cassini-Huygens mission will provide much new information on all of these moons, especially
Titan.