Not that I believe them, but every gram of glycogen in your muscles is surrounded by 3 grams of H20, roughly.  Say you're carb depleted, and you gorge on 3 lbs of pasta (haha yeah right, pizza).  500g of carbs now becomes 2000g of glycogen/h20, or ~4 lbs.  Math isn't perfect, but thats 7 lbs right there.