DND is not a simulation game
The first thing to note is that DnD is not a simulation game. It contains many rule problems that disappear if you try to simulate them.
The books I have reviewed do not deal with this issue, but the surest assumption is that conservation of mass, and the rest of physics, is actually ignored, at least once the magic is involved. For example, formulas such as enlarge definitely ignore all the implications of the square cube law.
The RAW answer is that the spell does exactly what it says and nothing more.
Spells do exactly what they say, no more and no less. Enlarge, the many other shape-shifting effects, and indeed all the spells I've examined do not say that they're pulling their mass elsewhere, so they're not doing it. The spell effect simply happens by magic.
Unless some books that I do not have contradict me, I feel reassured to say that the definitive rules as a written answer are that the entire explanation is "magical". with nothing more or less.
Warning: My access to 5th books is somewhat limited and in particular I have not read the 5th Dungeon Guide. That said, I think it's safe to say that DnD does not care about physics, especially when it comes to magic.