I found 3 perfect ideas to explain the mass of a shapeshifter:
<PERSON> - Hemlock Grove
This method is better to explain mass loss but can be used in both ways.
When the user decide to transform into a smaller been he left behind his transformation a mass of fresh flesh (remants of their last body) that he can eat later to turn back to his normal body.
This video shows the transformation in the serie.
Steam condensation - Shingeki no kyojin (Attack on Titan) and this
Well, in the serie isn't explained how transformation happen but fans have made a lot of theories.
This method is better to explain mass increase but if you want you can do the opossite effect.
When the user decide to transform it release an energy blast, fans believe that this persons absorbs the molecules of the air to make their bodies.
* This explain why after the transformation their bodies are so heat, chemical reactions usually produce heat.
* This transformation doesn't need nutrients to work, only air (maybe earth?) and energy to make "the transmutation".
* Because they would need a lot of food to get the energy they could use solar light. In the serie titans need light to move or regenerate, in dark they "sleep".
Virtual matter and teletransportation - Ajin
I don't remember if it is also a fan theory or not.
In Ajin there are some persons who can't be killed, if you kill them (e.g: split in two) after a few seconds one of the 2 part would regen in almost 10 seconds while the other would desintegrate (teletransportation? maybe...). This doesn't break matter conservation law (but yes momentum law, or I think...), but you want to make matter so this won't work for you.
Well, <PERSON> (inmortal persons) has more abilities, they are able to summon an IBM (Invisible Black Matter), don't ask about invisible because it isn't important here, also you can change black matter to standard matter in your mind.
They can make a creature of virtual matter. 337
Virtual matter isn't like normal matter, is normal matter WITH one exception: virtual matter was made out of nowhere -breaking law of conservation and other laws- so virtual matter need to be desintegrated in really short time to "don't break these laws".
Think that the universe is a human, if you break a law, it punish you (I am not sure but I think a black hole would be spawned of nowhere if you break a physic law).
Humans are living beens, and like animals can make mistakes, if you make something wrong your boss/teacher/parents would punish you but not instantly, maybe they take a few seconds or minutes to realice of your mistake. The universe is the same but it doesn't need seconds or minutes, it take yoctosecond (10−24s). By some [insert magical or sci-fi reason here] they are able to delay this and they can make virtual matter for some hours (e.g: in Ajin IBM can be used by 5 minutes).
* You don't need nutrients.
* You don't need solar light.
* Maybe you have only this two disadvantages:
* You can't stay to much time transformed.
* You need some coldown time. 337