Yes, but you made me look again*, just to be sure, and noticed the “preserve volume” option, checking it solved the problem… but I don’t understand why the head doesn’t have the same problem, having the same transform and origin…
I’ve just remembered I did something different when I parented the cap to the armature… instead of choosing “with automatic weights” I chose “with empty groups”… but I’m still don’t understand the whole picture, is it a bug or a feature?
Okay, that’s strange Parenting with empty groups will only add vertex groups equivalent to the deforming bones of the rig, but it will not add any weight into them. So you must weight paint it manually if you use it. But did the cap have some old veights or vertex groups left before you parented it into the armature? Sometimes that can mess up the deformation, I usually delete all vertex groups before rigging a mesh if I don’t want to use the old weights. And if the mesh has weights already, you can just parent it with keep offset like any other mesh, then add an armature modifier if not present yet and set the parent armature as the used armature in the settings of the modifier.
I’ll be damned… It was probably that. just to be sure I tested it:deleted the modifier, deleted the groups, parented the armature again with empty groups and it was ok…
Thanks again for the help and the lesson, really appreciate it!