Think cross bikes in general have shorter top tubes as the frame angle is higher for more bb clearance;my Planet XLS(I know not a Ridley*
) has much shorter tt and head tube.
It might be worth trying a bike fit with the Cannondale just to make shure,you'll then know for certain that it's the wrong bike or if you are lucky they might be able to fit you on it somehow.
When I got my first bike I did the same thing,bought online and realised it was the wrong size.A local LBS did a basic fit for me but confirmed it was really too big;I rode it for a bit but then got a frame the right size and
moved the parts over before selling the original frame.He also told me that frame sizing now is a nightmare and even the Pinarellos that he stocked had different sizing across the range.
*When funds allow I 'may' look at getting a nice Ridley cross bike