A chapter should be easy for the reader to digest one chapter at a time. 15K isn't short at all. Depending upon the font you use in your drafts, it could be up to 10 pages -- more if it's going into an average traditional size paperback novel.
When I first joined, I was frustrated with the character limit because the page count in Word for some chapters was 20-28 pages. In case you didn't know, that isn't good considering it's a Word document and would realistically be longer in a paperback print.
As I tried to find ways to break it up into Parts (3 parts in some cases), I realized the chapters had multiple topics going on, multiple scenes, multiple locations or plots within themselves.
I'm not new to writing but I can get wordy when my imagination starts going super fast. I just wanted to tell the story, not worry about breaks.
In separating the long chapters, I found they flowed much better and were much easier for the reader to place themselves within that scenario of that particular chapter. Whereas before, it was making them go all over the place in town, in the palace, listen to multiple conversations, etc... rather than just focus on the topic at hand at that moment.
The 15K character count improved my writing and has prepared me for more professional settings. I love it! I wouldn't change it for the world.
One thing I want to point out that I learned about years ago: Chapters are not necessary. Most readers prefer them, but it isn't required. There are books with no chapters at all (ex. Dune, Flowers For Algernon). When you consider that for Tapas, it frees your stress over needing structure. Just fill up the 15K and continue as you wish.