A few thoughts:
* Women gain water weight monthly; having that coincide with another
reason for water gain (or scale oddness) may cause the drastic
numbers.
* Size doesn't always predict water gain from glycogen, as glycogen is
more affected by muscle mass, liver size, and depletion.
* This is why I don't trust electronic or spring-based scales ;-)
* I can bounce many pounds per day from water retention changes.
That's why I graph my weight daily and only worry about the long
term trends.
To calibrate a scale:
Weigh both one and two known weights on the scale, verify they give
the proper numbers.
Weigh yourself with and without a known weight, verify that the
difference is correct.