> It seems likely to me that people who are speaking Indonesian as a first language (e.g. Jakartans) have a substantially larger vocabulary than those that are speaking it as a second language (e.g. Yogyakartans).
They are speaking Indonesian as a second native language, they will acquire Javanese and Indonesian at the same time but use Javanese in conversation and Indonesian when consuming TV, books and in school. So their grasp of formal Indonesian is going to depend mostly on education level.
As for colloquial Indonesian, due to the internet new colloquialisms spread much more rapidly to other parts of Indonesia. So it probably depends on how old said Yogyakartan is.