This is not a bug. The Marker Width of the ThingMark is used to determine the distance away form the camera of the Thingmark. So if the width of the ThingMark that is set in Studio doesn't match the real width, Vuforia will think the Thingmark is closer or farther way than it really is.
This is also true on other platforms, but projecting everything down to 2D to display on screen and continually tracking the ThingMark every frame hides this fact. On a HoloLens, the item really inhabits 3D space and by defaults, Vuforia tracking is turned off after the position is determined.
If you are trying to change the size of the model, you should scale the model. not change the size of the ThingMark.