He might vary day to day.
Where does the mass come from? Where does it go? Odo doesn't eat or drink AFAWK (except for show on a rare occasion), and he doesn't leave bits of himself behind (supposedly, those bits would revert back to helpless and mindless goo and rush back to the "motherlode" of Odo matter - although adult Changelings might not have this limitation).
Somehow, Odo is able to do to mass (or to its inertia and gravity) something no biological being can do. Then again, Starfleet certainly knows how to make the inertia or gravity of mass go away (and come back)... Perhaps Odo is just the biological equivalent of a gravity generator, instinctively and genetically, even if he doesn't know how to imitate other types of fantastic technology?
The way I understand it, he seems to shunt mass and pull mass out of subspace depending on how large or small a given morph is. Which makes sense if Changelings grow as they exercise their abilities--maybe they pull mass from subspace and keep it somehow as they exercise their abilities.
And yes, that does seem to suggest a Changeling could, over a long period of time, accumulate enough mass to surround a whole planet! Actually that WOULD be kind of cool