I experience this too. Big meals = lots of calories = lots of fuel for your body. If you don't use up that fuel during physical activity, then your body just sits there with a ton of energy, without an outlet. It's like a car with a lot of fuel. You could use up the fuel by driving somewhere, but if you're just going to idle in the driveway, then the only way that car can burn fuel is by revving really loud. Your pounding heart is the revving engine.
That's just my interpretation. It still bothers me, especially when it combines with bloating/gas/reflux, and my chest hurts at the same time. Breathing also becomes uncomfortable.