Ball pythons can gain or lose weight at any time of the year.
It is all based on how much they are eating (and to a much
smaller extent, how much they drink). If they are eating a
lot during the winter, they will gain weight. If they go on
a 6 month hunger strike (which isn't uncommon), they will
lose weight. I've had females pack on a 1000g during the
winter. Perhaps some of them get triggered into thinking
they should be breeding soon and need to put on additional
weight (of course ideally they would have been prepared for
the season ahead of time...).
If an animal is not eating or drinking, they cannot gain
any weight. Even during egg development, they cannot
'spontaneously' gain weight. During that timeframe, fat
reserves of the animal are being converted into developing
follicles/eggs.
Hope this helps...good luck.
Chris