If you think about it, SW food is quite heavy and bulky. Lots of vegetables, for a start.
Now if I sit here and eat 1 kilo of carrots, I will get up 1 kilo heavier. But tomorrow, the carrots will have gone through the digestive process and most of that weight will have gone on its merry way, leaving very few calories behind.
If I sit here and eat 1 kilo of cream cakes, I will get up 1 kilo heavier. But tomorrow wouldn't bear thinking about!!
(for any experts here, forgive the totally unscientific nature of this description)