Yes, dice containing a hidden insert can vary considerably in weight, depending on the content. In general, the more loading material, the heavier they are; lead is heavier than iron; those with hollow spaces inside are lighter. All these dice are 18mm size.
All dice in the 16mm (mis-spotted) range are the same weight: approximately 4.6 grams.
Approximate weights of 18mm (hidden insert) dice range:
Matching Regular: 6.2 grams
Face Loaded (with lead): 13.5 grams
Edge Loaded (with lead): 11.2 grams
Corner Loaded (with lead): 10.8 grams
Iron Core: 16.1 grams
Face Loaded (with iron): 9.5 grams
Edge Loaded (with iron): 8.5 grams
Corner Loaded (with iron): 8.3 grams
Overloaded: 9.0 grams
Float: 4.1 grams
Reloader: 7.2 grams
Flip Dice: 9.4 grams
Magnet in One Face: 7.2 grams
Magnets in Two Faces: 7.4 grams
Magnetic Core: 11.3 grams