Is a Returned's Ability to Change Appearance Like Feruchemical Pewter?

[Trusk'our he/him]

When a Feruchemist taps their metalmind, I'm pretty sure that their weight doesn't change, even though their muscle mass does. When a Returned shrinks or grows, does their mass/weight stay the same?

[Quantus he/him]

The Returned ability is closer to proper Shapeshiftinging than any of the (more focused) feruchemical metals. You can see it in the lesser Royal Locks, which WOB says could be stoked up to the Returned's shapeshifting.  Im pretty sure they actually gain Matter  and thus mass when they do their change, more like feruchemical pewter that adds muscle mass than storing weight that is some sort of Hoggs field effect that kinda breaks physics.