I think the thing with materials is that even with demand being low relative to instructional capacity, admissions likelihood is going to depend how you stack up vs the applicant pool. With a small applicant pool, in an engineering major that is less well known I suspect there is more variance in the strength of the applicant pool year-to-year than a larger discipline like mechanical. For all I know the applicant pool for materials is if anything in some or all years stronger than say for mechanical. I believe the admission statistics (i.e. incoming GPA) for individual departments at Cal Poly are not publicly known. According to the materials department at Iowa St. for example, half of the students are in the honors college (overall, I think this is a good thing) so it might be that while it is a small corner of the engineering college, better students may be over-represented relative to the rest of the engineering college (again, overall a positive in my view if that is true). At the end of the day, the only thing you can do is apply and see how it shakes out.