iPhone 6S: 138.3/67.1/7.1 = 65.9 cc the mini is just barely smaller.
iPhone 4S: 115.2/58.6/9.3 = 62.8 cc smaller than the mini.
Treo 650: 113/59/23 = 153 cc which is about the same volume as a Galaxy Z Fold 3... the displacement was so uncomfortable, people often used hip holsters for the former.