TweetTakes time to put on BF. 15-18% is also a pretty big gap. I am the same way. I am skinny and do mostly cardio so I stay between 11-15%. Eating an extra 9,000 calories in a week if all else stays the same might cause you to pick up a LB of fat but our bodies resist change. They want to remain in optimum homeostasis. So after you eat your gallon of IC you body says back to normal and that LB slowly melts away.
Takes chronic overeating to gain weight. If you ate a gallon a week for 2 months I bet you would see a significant diff.