There's little more to it than just TT length and head tube length. Differences in the seat tube angle can make the reach longer or shorter, just like changes in the TT length. The effect is about 1cm per degree. A 55cm TT with a 74 degree STA would be about the same as a 56cm TT with a 73 degree STA and no, I did not get that backwards. Steeper STAs increase the reach.
Also, when comparing the head tube length, the headset stack height needs to be included. Conventional headsets have 25-35mm of stack height and integrated models are usually in the 8-20mm range.
I pay no attention at all to frame size numbers and never compare them between brands.