Large differences in admission selectivity, together with large differences in faculty resources, probably do correlate with significant differences in the pace and rigor of instruction. That doesn’t necessarily mean anyone has taken the trouble to measure (and publicize) related indicators for schools that interest you. There are many hundreds of colleges. They are complex institutions (with great variation even among their own courses/departments/professors).
Have a look at the ranking methodologies. AFAIK, none of the major rankings consider any factors directly related to course rigor. The National Survey of Student Engagement has surveyed many factors related to course rigor, but deliberately avoids using their surveys to rank colleges.