genetics, mainly glycogen storage imo
i get absolutely tiny when cutting down, and get huge quick when bulking up again... i also usually lose/gain 15-20lbs within the first 2-3 weeks
my buddy for example doesnt lose shit when he starts to cut and doesnt gain shit when he starts to bulk (with regards to water)
he can hit test, deca, dbol and eat 1000g carbs a day (he actually does) and doenst gain more than 4-5lbs water weight
put me on that stack/diet and id look like the stay puff marshmallow within a month and gain 40lbs water
even when he gets fat he just has to lose 15lbs of pure fat and thats all the weight he loses
for me i would gain at least 15lbs of water with 15lbs fat (but id lose these 30lbs within the same timeframe)
it just makes you much more depressed quicker when cutting if youre one of these heavy weight gainers