1.) The statistics you state are for the current Mars, stripped of its atmosphere.

2.) I propose that the former mass of Mars may have been much higher (like 20 to 50%), and that this mass was gas and condensed gases, namely water.

3.) The gas giant planets do not need a rocky core in order to form. Mars would have been in a cooler accretion zone than Earth.

Last I heard, it was still unknown if the gas giants even had a rocky core (correct me if I'm wrong). I imagine they do have Mars to Earth size cores, however.

The Gallilean moons do not have much atmosphere largely for the same reasons that Mars no longer does, small mass and radiation exposure. Furthermore, their accretion disks were small compared to that of Mars, since they were formed from material orbiting Jupiter, not the sun. Titan, a body smaller than Mars, does have substantial atmosphere. Other moons may even still have liquid.

I will suggest a wild speculation that perhaps Mars did reside near where the asteroid belt is today, but that the loss of mass (while retaining momentum) allowed it to move to its more distant orbit today. Anyone know how much mass loss would be needed for this?