Canyon Endurace - Sizing?

[chris1012]

I am 171.5-172cm in height (bare foot) and 77.5-78cm inseam.
I bought the Canyon Endurace CFSL 8.0 di2 size S on the recommendation from my previous bike fitter who advised my previous Cannondale CX frame was a good match for the previous bike geometry/measurements.
I also checked the measurements of the size S on Spoke99 in comparison to my Cannondale which showed the size S was a much better/similar match than XS.
I have been on a couple of rides since I had the Endurace setup on Friday and my lower back is feeling quite uncomfortable during the rides.
I have checked Canyon's website and based on my measurement's they recommend a size XS however if I change my height from 172 (same inseam of 78) to 173, it recommends a size S.
I have made the same measurement's as my previous bike for my saddle height and reach.
Any one know what could be causing the discomfort?
Previous bike measurements attached.

 

[Buck]

The sizing is a very personal thing because it’s a complex mix of inseam, upper torso length and flexibility plus other factors I.e. injuries etc.

im not sure what length cranks yours has? Mine has 170s. My old bike has 172.5

im 170cm tall with an inseam of 76 and the XS on my Endurace is perfect. I was a bit worried as the cockpit is a one piece so no changing stems!

Your lower back is most likely feeling the “stretch” of a slightly longer top tube. Have you checked your saddle position and tried moving it forward say 1cm. Relatively small adjustments can make a big difference.