Direct democracy is a process in which citizens participate directly in decision making processes. Of course, if one had a large amount of people with lots of different ideas, there would be a variety of issues. Firstly, it would be harder to represent everyone as more people would need to voice their opinion, making it a slow process. Secondly, not all people are equally specialised or educated to deal with certain topics.
So yeah, one could say it works better in small groups where everyone has an equal say and can make decisions quickly and efficiently.