You can’t just add students without infrastructure and facilities to support them. Merced has an intentionally overbuilt core for its current size. The campus is also surrounded by vacant land and a very welcoming local government - making adding students - and the infrastructure they require - faster and cheaper. Housing and space near all of the other UC campuses has grown scarce.
Here’s an article about how UCSD is dealing with a their rapid growth.
https://www.sandiegouniontribune.com/news/education/sd-me-ucsd-enrollmentgrowth-20181030-story.html
UCSD