How should I choose between Stanford and Harvey Mudd for studying physics?

For the priorities you named, Harvey Mudd is likely the more fitting choice for physics. Its small, undergraduate-only environment makes sustained contact with physics faculty a normal part of the academic experience. Mudd also has a strong STEM culture and a rigorous common core that builds mathematical, computational, and engineering fluency alongside physics.

At Harvey Mudd, faculty research groups are built around undergraduates, so students can more readily get to know professors, join projects, and receive detailed mentoring. The tradeoff is that the research menu is narrower: there are fewer labs and fewer highly specialized subfields available on campus than at Stanford.

Stanford offers a much larger physics ecosystem, including extensive campus research and access to SLAC National Accelerator Laboratory. That can be especially compelling for a student drawn to areas such as particle physics, astrophysics, condensed matter, quantum science, or large-scale experimental work. Stanford also makes it easier to combine physics with fields such as computer science, engineering, mathematics, or policy.

The experience at Stanford can still be highly personal, but it requires more initiative. Early courses may be larger, and research groups often include graduate students and postdoctoral researchers, so building a close faculty relationship may take deliberate outreach. A student excited by navigating a broad, ambitious research environment may thrive there; a student who wants close professor access to be built into daily college life will probably find Harvey Mudd more satisfying.