I recently ran a keto diet, mainly to see what it was like. After 6 days I ran a 24 hour carb up, and then ran no carbs a further 4 days and by the end of it I experienced a 3/4 inch reduction on my waist and about 7lbs lost. Now I know most of that wasn't fat or muscle so I'm thinking it was water weight. My stomach went from wobbly to pretty tight in that time span. I'll add that I consider myself an ectomorph by nature and I'm 180cm and 82kg normally but about 78 after keto.
Do some people just retain water more than others? If so, are we susceptible to retaining water regardless of what type of carbs we consume? For the record I normally drink about 3 litres of water a day.