You know... It is theoretically possible for him to eat the planet, if he keeps eating and condensing until he has a similar mass to it. Of course, the planet will probably notice him if he gets too heavy, not to mention that for all we know the planet itself could be highly condensed, and there's the original problem of eating before killing (Although, I presume that by now he has eaten thousands of microscopic demons, which means their consciousness is too small to notice, or he kills them through highly acidic/toxic digestive juices.)
Also, he can't have been the first demon to have tried to return to an area populated by smaller demons, which makes me believe that there is a REASON to this. Will we perhaps discover this reason soon?