At around 6 mo, many babies are ready to drop the catnap and transition to 2 naps. If your DD is ready for this, then her naps will likely lengthen on their own once her body has adjusted. Judging from what you've said, though, she may not be ready just yet. If she's happy with short naps and is putting herself to sleep easily, then I wouldn't mess with it for now. Tyler took short naps for a few weeks before we transitioned him to 2 naps. We knew it was time when he started resisting going to sleep big time because he needed more awake time. Once there weren't enough hours in the day to fit in all 3 naps, we moved him to 2 naps.
Also, keep in mind that the daily amounts of sleep are just guidelines based on averages. If she's happy and content, then chances are she's getting as much sleep as she needs!