I don’t know much about grad school but at almost all universities I have looked at there’s fewer students attending the grad school than undergrad. Is this because there are fewer places in grad school which makes it more limited and harder to get into? Is it because there are fewer students applying? If we take a university like ASU for example, how many students are admitted to the undergrad computer science program each year and how many are admitted to the master’s program in computer science?
Graduate programs require a lot more time and effort and money on the part of the faculty per student than undergraduate programs do, so there simply isn’t as much room for graduate students in a program of a given size. Of course, there are also a lot fewer applicants so it works out.
In addition to what boneh3ad already said many (most?) graduate programs don’t release acceptance data (either number of applicants or their acceptance rate).