There is no serious upper page limit on a thesis and you can truly spend the extra time to make your thesis stand out.
- Put the results of your earlier papers together in a common framework and add some new results you never bothered writing up. (Harry Buhrman's 1993 thesis has a large collection of results on exponential-time computations that I still often consult.)
- Take the time to expand the proof of complicated results to the right amount of intuition and depth. (For many years Madhu Sudan's 1992 thesis had the best write-up of the proof of the PCP theorem.)
- The initial chapters of your thesis can serve as an introduction to a relatively new research area, (Michael Kearns's 1989 thesis gave an early broad overview of computational learning theory.)
- Or give your own impressions of a more established field (Scott Aaronson's thesis expounds on his views of quantum computing.)