I think you've answered your own question. I tend to travel quite heavy (big suitcase, duffel with dirty laundry and a laptop backpack) and I've never had any issues. So I suspect that traveling light tends to make you "different", as well as I suspect that you're the first one out the door. That gives CBP a little more time to look you over and make a call because there is not a line of several hundred behind you.

There may be other aspects in how you're dressed or perhaps some age or racial profiling.

A friend of mine used to do MRs to AMS quite frequently. He would take a very small gym bag with a change of underwear, some toiletries and a couple books. I asked him how many times he got the rubber glove treatment on return from an overnight to AMS. He told me never -- he didn't fit the profile of someone who was a drug mule. I always found that hard to believe, but he was an overweight, balding middle-aged white guy. (Come to think of it, that's me as well.)