Interestingly i felt like my last replacement was actually a downgrade.
Had the X (10) for years and it was great and compact but the face-id broke, upgraded to the 13 (because i didn't need anything in the 14/15) and now i have a phone that's too bulky, and not as comfortable to hold.
The X was simply thinner, more rounded and a way better experience, also i feel almost no difference doing day to day stuff.
I too prefer the more rounded exteriors. It feels better in the hand. I think it's just a cyclical fashion choice, was an adjustment period when it was iPhone 3 (rounded) to iPhone 4 (squared) too and it more noticeable when not cased.
It's interesting that you perceive the X to have been thinner. It's actually very slightly thicker (by 0.05mm) than the 13. The 13 is longer (by 3.1mm) and wider (by 0.6mm) though.
Had the X (10) for years and it was great and compact but the face-id broke, upgraded to the 13 (because i didn't need anything in the 14/15) and now i have a phone that's too bulky, and not as comfortable to hold.
The X was simply thinner, more rounded and a way better experience, also i feel almost no difference doing day to day stuff.