It has been suggested from time to time, that the breast sizes of the individual OS-tans represent their RAM size. Because Windows XP is considered a memory hog due to its increased resource consumption, XP-tan is incredibly well endowed (and she has no qualms about getting "upgraded" from time to time). 2K-tan normally rates as a close second, whereas the DOS pair are at the other end of the spectrum.
Another theory states that the breast size of an OS-tan represents the overall "fanciness" of their graphical user interface. Since XP was designed with bells and whistles, she has the largest breasts, but DOS, being no more than a command prompt, is at the other extreme.
Japoneses... aposto que os vírus que comem os OS são desenhados cheios de tentáculos... 
Haha
Isso tem algo a ver com tirinhas de internet?
Eu ainda não consegui entender a origem de um pensamento tão... tão...