Here are the two bikes at the extreme sizes of my current "fleet". The smallest in front and the largest in back.



I measured them just now. Admitting there is likely some measurement error, I get:

Small bike (Cannondale): seat tube ctc 51.4 cm, top tube ctc 55.0 cm, seat tube angle 75 deg, head tube angle 74 deg, stem 110 mm, center of saddle to bar top 66.0 cm (horizontally), saddle surface to pedal upper surface at lowest position 89.6 cm, weight as shown 18.9 lb (includes spare and inflator)

Big bike (Peugeot): seat tube ctc 56.5 cm, top tube ctc 58.4 cm, seat tube angle 73.5 deg, head tube angle 75 deg, stem 85 mm, center of saddle to bar top 68.0 cm, saddle to pedal at lowest position 89.9 cm, weight as shown 26.3lb (fenders, pump, lights, full water bottle and all, this is as-ridden).

It looks to me like a 130 mm stem on the small bike, plus raising the saddle 0.3 cm, would get the cockpits to about the same size (saddle to bar distance) and I wouldn't have to push my saddle much further rearward. I rode around today on the small bike and, when pedaling higher cadence, my rear doesn't want to be any further back than it is. Only when pushing a hard gear, e.g. climbing, does my rear migrate to the aft end of the saddle.