There's typically a great deal of flexibility in your thesis presentation, and as long as the lengths don't disturb you or your committee, there is no reason to force your work into some [Procrustean bed][1] of perceived length balance.

In my own thesis, the number of pages per chapter were approximately: 12, 4, 8, 8, 36, 12, 44, 16, and 6 pages, respectively.  While there may have been problems in the document, chapter length was not one that either I or anybody who encountered it ever had any complaint around.

In short: let the sections be the length that is best for them, and don't worry if they are different.


  [1]: https://en.wikipedia.org/wiki/Procrustes#Cultural_references