If your shapeshifting doesn’t break thermodynamics, then you’re not able to shapeshift into anything smaller or larger than yourself.
(Though, perhaps, you could stretch it a little by ‘cheating’ it – become larger by shifting into a hollow shape filled with air, become smaller by shifting into a shape filled with very dense material. But there are limits to that – too big, and you’ll float away like a balloon; too small and the ground/floor beneath you won’t be able to withstand the pressure, so you’ll sink down into it – and you’ll never be able to change your overall weight/mass, which might compromise your disguise in some situations.)
If your shapeshifting doesn’t break thermodynamics, then you’re not able to shapeshift into anything smaller or larger than yourself.
(Though, perhaps, you could stretch it a little by ‘cheating’ it – become larger by shifting into a hollow shape filled with air, become smaller by shifting into a shape filled with very dense material. But there are limits to that – too big, and you’ll float away like a balloon; too small and the ground/floor beneath you won’t be able to withstand the pressure, so you’ll sink down into it – and you’ll never be able to change your overall weight/mass, which might compromise your disguise in some situations.)
Simple. When you shapeshift, your extra mass gets shunted into Z-space. When you need extra mass, mass is borrowed from other masses in Z-space.