I was an undergraduate at Caltech and helped with some admissions work. There is no bias towards admitting females. The student body is extremely small (200-250/year) and the student pool extremely self-selective. The 4 year graduation rate is troublingly low enough (people will transfer or drop out) that admissions will not admit people if they can’t meet the academic bar (and the curriculum is structured so that the first 1.5 years regardless of major require the same classes). They do however emphasize top-of-funnel outreach for women to try to get more to apply.