Well, it's been a long time since my PhD in Applied & Theoretical Caffeination, but I can still recall the basics and so, in the interest of Bringing Science To The Masses, permit me to explain the phenomenon.
It's a noted scientific fact that the mug size decreases in inverse proportion to the need for coffee, due to quantum irregularities. The phenomenon was first noted by Isaac Newton, in fact, who was in dire need of a triple espresso after an unfortunate apple-related incident left him lying flat on his front for three weeks. However, while his work on mug size led directly to both his Laws of Bowel Motions and a severe case of caffeine withdrawal, he was unable to explain the reason for the events he observed.
Many people believe that the breakthrough in understanding the coffee/cup/need relationship was best postulated in Folger's Caffeine Uncertainty Principle, which reputedly came to the great man after a heavy night on the Jack Daniels. The Principle states that because the act of needing coffee changes the quantum state of the coffee itself simply by being present, we can either know how much coffee is in the mug or how good it tastes - but not both. A purist would, of course, note the fundamental contradiction inherent within the Caffeine Uncertainty Principle, though lack of space prevents me explaining it here. However, many in the field now agree that Folger would have produced a better Principle had he not been hungover when he postulated all over the page.
However, some caffeticians believe that this Universal question can better be explained by the groundbreaking work of none other than Albert Einstein in his 1918 General Theory Of Javativity, which states that:
where c= cup size, N= need for a brew and e= The Eurgh Constant*, brought about by not having coffee and believed to be the largest 'real number in existence**.
Now clearly, I don't have to tell you that Einstein's theory leaves significant gaps for the thinking caffetician. The theorem makes no allowance, for example, for the significant effects of spacetime on the surface area of a cappucino, for example, which many believe was Einstein's greatest error (after his choice of hairdresser).
That said, Einstein could not have been expected to predict the later work of Professor Maxwell House of Princeton, who proved that coffee exists in multiple quantum states, acting like both a wave, a particle and a good smack around the ears at the same time, and whose seminal work (thankfully just missing his mocha when he released it) on Latte Theory is still being studied for both insight and any sense at all.
So you can see from just this simple primer that the question has taxed some of the finest minds in history, and we still have much to learn about the physics of the coffee mug. However, mine is now empty so I shall refill it before it reduces to subatomic size. Join me next week, when we'll be investigating the theological schisms caused by Marmite sandwiches through history.
* This footnote intentionally left blank.
** The Eurgh Constant is defined as the square of the cube of the likelihood of you getting a kick in the teeth if you don't bring me a double espresso right now, multiplied by the Fine Structure Constant and divided by the number you first thought of. Plus 42, obviously. The equation is çɞʧE³. You're welcome.