The technical bit - if you take a litre of water and freeze it, it weighs the same as it did when liquid - HOWEVER, the molecules in it expand and the volume of it increases when it is frozen. So a litre of water would be bigger than a litre of water in terms of volume and density, but would weigh the same. However, because of the change in volume and density, it would float, like ice cubes in a drink.
When an item of food containing water defrosts, some of the water will drain away from the product as part of the defrosting process, what with it being liquid and not solid any more - so 100g of frozen mince, for example, once defrosted would weigh slightly less because it now contains less water than it did, but it wont be significant enough of a difference for you to worry about. If you defrosted the meat in the bag you sealed it in for freezing however, the overall weight would remain the same because none of the water content would be allowed to escape. Overall the mass does not change.