Good question!
Sometimes, while reviewing your PDF, you notice some of your chapter titles are set to a smaller font size than the rest. Even though you have customized your theme and set your Chapter Title font size, you are seeing your Font size appearing differently.
Within your Chapter Settings, you can adjust the size of the Chapter Title to create visual hierarchy or accommodate longer words within your PDF.
To change the size of these chapters, go to that chapter in your book and click the gear icon below the title of the page. Ensure the option, "Use Smaller Chapter Title”, is unchecked.
Hope this helps!