Plan chapters after deciding the scope of the book. Chapter length will be in part determined by the amount of material to be covered in a subtopic. Decide the level of detail required in advance.
Make a chapter longer by advancing more theses that need support material. Keep the new issues you raise relevant, and try to avoid adding detail for the sake of it, as this intention will be detected by the reader and you risk losing credibility. By covering more real issues you can maintain a conciseness and clarity in the writing, while still adding length.
If your writing is too unevenly spread, or if you are writing a lot on aspects that interest you personally and tending to ignore topics that do not, then adopt a plan to avoid this. You can allocate, for example, seven main subtopics for a chapter, then have four headings within each topic and two or three paragraphs per heading. Once you have this on paper you will have a clear picture of all the material in front of you and what needs pruning and what needs further development to maintain a good length.
Use the length of chapters to establish pace to a novel. Short chapters, perhaps with startling or vivid action, will increase the pace; longer ones ease it off.
Get to know more about the characters appearing in the chapter. The simple fact is that longer chapters require more material, and the writer needs to research the setting of the book or invent more things about the characters appearing in it. Consider their background, life history -- everything that makes up the back story -- and then write more details to create more length.
Decide the length of chapters according to your readers. Young readers will need shorter chapters because of their limited attention span. Adults, on the other hand, expect a chapter to be long enough to justify itself as a separate unit, but not so long and sprawling that it loses its sense of direction or identity. Add length with solid description -- about character or place -- that is involving for the reader. Add more action and events if they illustrate and develop the themes of the book.