Gmail's webclips feature alerted me to the availability of Edmund Landau's PhD thesis in English translation. The thesis is 13 pages, which is substantially shorter than any PhD thesis in CS I am aware of.
I suspect that PhD theses in mathematics are, by and large, shorter than those in TCS. What is the shortest PhD thesis in either subject you are aware of? (In the case of mathematics, let's restrict ourselves to modern times, but if anyone knows of a thesis that is shorter than Landau's I'd be interested in knowing the details.)
Here is my own contribution. Off the top of my head, I recall that Jay Loren Gischer's PhD thesis from Stanford (year: 1984; topic: the theory of pomsets; supervisor: Vaughan Pratt, see also here) was between 45- and 50-page long.