Reminds me of when I got my first job at 17...I worked at a large department/grocery store. My job primarily consisted of pushing karts from the corrals back to the store.

In an 8 hour shift, I likely walked ~15 miles, with half of that time pushing up to a dozen karts. For lunch, I'd go to the McDonald's and get a Super Size (Since this was when that still existed) Double Quarter Pounder meal with a Coke. I'd chug the whole coke and then refill it. This meal was easily 3,000 calories, and I'd eat it 3 times a week.

After about two months on the job, I'd STILL lost about 5 lbs.

Polar explorers Fienes and Stroud were eating locks of butter and still losing weight on their sled pulling expedition. It was estimated that they were buring up to 11,000 calories a day.

Your McD's meal was probably closer to 2000 calories, even with the giant Coke. For a short period I ate one meal a day, even when I thought I went over maintenance with a massive fast food treat, I'd still be 4-500 cals under if I was active that day.