Brumation is a dormancy period for reptiles that is similar to hibernation. It's different from hibernation in the metabolic processes involved. They can go months without food, but still need to have access to water. I'm talking access to water for snakes being kept above ground....outdoors the snakes are getting moisture from the ground.

With their food intake stopped, I would say no, they aren't growing, if anything they would be burning fat which keeps them going during the winter months. Since they need heat to be able to digest food they are pretty much just dormant. That's my take on it....hope it helps.

Visit these sites to help you understand more....
http://en.wikipedia.org/wiki/Dormancy

http://lllreptile.com/info/library/care-and-husbandry-articles/-/reptilian-brumation/