Jogging and pecs answer please?

Since I started jogging and biking I noticed my pecs look a lot smaller??? Why is this I'm hitting harder weights and have good rest, nutrition and protein. Is it because the jogging has burned some of the fat from around my pecs? And it won't be burning my muscle I'm only doing 20 min jogs 5 days a week you need to do a lot more to be cutting on muscle.