Is it okay for a thesis to have sections of very different lengths?
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 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.