A linguist noted this principle in Ted Chiang's novella "Story of Your Life", during a discussion about Fermat's principle of least time. I am not a native English-speaker and I have not found any info about the above principle. Does "Louise's principle of least closet space" exist? What is it about?
This "principle" doesn't actually exist, except in Gary's mind. He's using it as an abstract example of reductionism, the way in which physics tries to reduce everything to simple rules and laws that can then be used explain more complex concepts.
Based on other examples of the "principle of [the] least" (1, 2, 3), we can reasonably assume that principle of least closet space relates to the way in which Louise tends to store items in the smallest closet that will handily accommodate the item. Having shared an office with her for several weeks, Gary would be well aware of this quirk.