the semi-scientific term snowball effect is used to discribe any object that gradually gains volume, such as when a snowball rolls down a hill. it is a term commonly used to describe people who have large weight gains.
the snowball effect can discribe young people who gain weight, mostly due to the natural cycle of growth. for example, while in middle school and just starting puberty, a preteen may be skinny and lanky. but as the reach their teen years, their body mass will gradually increase.
the snowball effect can also (although more harshly) apply to an adult who bulks up due to excessive eating or disease.
my girlfriend really succumbed to the snowball effect. when we were young kids in middle school and were just friends, she was really skinny and weighted under 90 lbs. while we were together in our high school years, she had a little meat on the bones, and was gradually getting bigger. by the time she was a sophomore in college, the snowball effect had caused her to weigh 145.