starting legs

I was talking to a friend today at the gym and he was telling me about a little about legs. Now I usually never workout my legs(cause i have bad knees), like maybe once a month and when I do I dont work them out that hard. But the last few weeks I have been noticing my upper body is getting to big for my lower body. He told me tho if I start working out my legs I will probably put on some good weight. Is this true, would I be able to maybe put on like 10-15lbs in a few months if I started training them like once a week along with a good diet.
thanks bros