Odo, the shape-shifting constable on DS9, can change form and shape, including shrinking himself down to the size of a mouse or a drinking glass. In the case of the drinking glass, Odo also can mimic the glass’ mass.
When Odo shifts from his humanoid form…a form with the mass and volume of a humanoid…where does all the extra volume and mass go?
Odo’s natural form is a liquid, and liquids take the form of their container . . . but gasses expand to the shape of their container . . . ignoring the obvious “he must be full of hot air” joke . . . and ignoring Cecil’s column about whether frozen milk actually weighs more than liquid milk . . .
Well, since his name is a shortened form of “odo-etal”, the Cardassian word for “nothing”, he shouldn’t have any size or mass at all. So any time you see Odo on the show, it must be your imagination. He’s nothing. He’s not really there. That’s all you need to know.