Formatting .MOBI for Kindle

I am trying to make my chapter titles appear larger. Currently, I have the chapter titles, along with a small line underneath, included directly in my text document where the rest of the chapter content resides. Then I just use the “As Is” option in the compiler. This works fine for PDF/Word formats, but for the eBook the chapter title is too small.

I can’t figure out how to make the chapter title larger.

I’m probably not following the best practice in that I’m putting the title directly into the main text file for each chapter, but I’ve not found a better way.

Any examples or advice are appreciated. I’ve looked around on youtube and not found anything that explains it.