A lot of people are probably familiar with this sort of handwavium in fiction when it's asked where a shapeshifter's extra mass comes from. Most choose to ignore that question. In works that pretend to be slightly more credible, they simply state that no extra mass is being created at all and that the shapeshifter is in fact merely grossly obese. Or at least they would be, were they not using their power to simply make themselves look like a 200lb man who actually weighs 500lbs. Presumably this is done by eliminating a lot of the empty space either in the human body or between the molecules themselves.
But this begs the question. How much mass can you actually handwave away like this before it becomes really obvious that this 150lb woman is actually a 700lb shapeshifter in disguise? Density is still a thing, and at some point the surface area to density ratio of all this compacted flesh and biomass is going to make the shapeshifter's legs sink into the very ground they walk on, no matter how careful they are to conceal their true weight.
When would this happen? What's the realistic limit to how much mass a shapeshifter could conceal in a humanoid frame before the extra weight becomes obvious? I'm assuming it would have to be greater than 500lbs but I'm not sure.