Phobos and Deimos are small, and are obviously either captured asteroids, or were formed from ejecta of impacts on Mars. Phobos has been speculated to be 25-30 percent empty space, iow, a pile of aggregated debris, basically a pile of rubble based on a one or a few largest pieces — much like every other asteroid that has been studied from close up.
The radar study of Phobos was MARSIS, and was by the ESA,, not the Russians.
There was a Russian (alleged) scientist who started claiming back in the 1950s that Phobos was moving irregularly. Naturally, at that time the mass of Mars wasn’t accurately known, and neither was the mass of Phobos for that matter. Here’s fringe website Rense’ page on this:
http://rense.com/general20/eisenhowerWH.htm
Better:
http://www.universetoday.com/58923/could-phobos-be-hollow/
And oddly enough, Wikipedia has the inside scoop:
http://en.wikipedia.org/wiki/Moons_of_Mars#Mars_moon_hoax
Hoagland's take on the subject:
http://www.enterprisemission.com/Phobos.html