Okay, I'll try to explain simply.
The shortest cable you can really have with full length needles is about 16". When the cables get shorter you must make the needle shorter. There are a few out there, but they are different to knit with and many people don't like them.
They still wouldn't solve the problem of the top of a hat though. As you decrease and get fewer stitches it starts stretching the fabric and you either need to change methods or knit flat. If you want a hat with the smooth rounded top you will need to use another method to work the top.
Options if you don't want to do that are to either decrease as far as you can and then stop and gather the stitches or just stop and gather the stitches without decreasing. This is actually one method some patterns use rather than decreasing. Knit as far as you want and we can help with the finish when you get there.
Does that help?