OK doing a quick search for the Preserve Volume in Blender is this link from the Blender Manual  https://docs.blender.org/manual/en/latest/modeling/modifiers/deform/armature.html  Seems you find the option in the modifier stack under the Modifier Stack for the Armature also I noticed you are using a Blender version that looks like a custom build have you tried using just default Blender version there might actually be conflicting mods with in that build you are using that might be the root cause of the issue you are running into.

Lobo3433

Blender Maya & 3D Forum Moderator 

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