UIUC has about 32K undergrads. OP’s S was accepted to UCSD, not UCD. Changing majors at any of those schools is limited to those that are not space constrained, and many have GPA requirements to make the change as well. Yes, Kzoo will have smaller classes on average than the state schools, as well as easier access to profs (at least for the first year or two), and Kzoo is a fine, reputable and rigorous school in many academic disciplines.
For OP, linking to a tableau of the National Science Foundation database for doctorates and their undergrad school for 2016-2020: Doctoral recipients by bachelor's degree-granting institution, 2016-2020 and 61 years of data: Baccalaureate origins of doctoral recipients
Source data here (somewhere!): Doctorate Recipients from U.S. Universities: 2021 | NSF - National Science Foundation