The small diameter premolar provides more retention because it offers a smaller occluso-cervical dimension, resulting in a greater surface area relative to its diameter for resistance to dislodgement. When height and taper are constant, reduced diameter enhances the ratio of surface area to perimeter, improving retention. This principle is crucial for prosthodontic preparations where retentive form depends on preserving maximum tooth structure around a smaller cross-sectional area. Hence, a smaller diameter tooth like a premolar retains fixed restorations better than larger diameter teeth despite identical height and taper.

Reference: *Fundamentals of Fixed Prosthodontics*, Herbert Leo Shillingburg, 4th Edition.