well I didn't explain myself overly well. Here's a second attempt, which probably won't be any better
if you take a sphere that's 1 inch in diameter and has a gravity factor of (making shit up here) 1, and you then increase the diameter of that sphere without increasing how much stuff makes up the sphere (i.e. you put more space between the atoms of the sphere) so that it's 2 inches, the gravity factor stays at 1.
Density does = mass/volume, but gravity does not = density.
Gravity is simply the measurement of attraction between two objects.
So if I'm orbiting the sun at the distance of Venus, and the sun expands without gaining or losing mass, and assuming the sun does not engulf my orbit, then I'll stay in the same orbit. If the sun engulfs my orbit and spreads out to, say, Jupiter, and I'm in a suit of unobtanium which allows me not to be burned up by the expanded sun, then I'll break orbit. Why? Because some of the mass that was pulling me <-- that way is now over --> there. My velocity didn't change, but the force pulling me inward has decreased (and if you wanna get REAL picky, there's now more force pulling me outward, namely the part of the sun that expanded beyond my orbit). So now my velocity will pull me out of orbit.
Wow. That's tangled. Did it make ANY sense?