slow metabolism
That doesn't make you constantly gain weight *that* fast. Even with a slow metabolism (and I'm not one of those skinny guys who can eat an entire pizza by themselves and remain skinny as a string bean, either), you still plateau at a point at which the slow metabolism will burn what you feed it. Oh, you may keep gaining every year (my parents and one of my grandmothers are like that), but not by leaps and bounds every month.
Dammit, why couldn't my father have inherited whatever "fast metabolism" gene his mother had? She was light as a bird for as long as I knew her despite the proverbial "meat and potatos" meals that made my grandfather somewhat overweight.
On the other side of the family, it was Grandfather who was the skinny guy and Grandmother who was the heavy one. The gene pool must have been stirred badly. :(
