Capacitors in Series - common node
The +/- represents the voltage relative only to the capacitor's other plate only.

If I am standing on your head, our feet are always below our heads, but that doesn't mean my feet and your head can't be at the same height.

The voltage potential at the bottom plate of C1 is not different than the potential in the top plate of C2. This is different than the charge on each plate.

If I have a large and small air tank and connect them together, their pressure (voltage) will be the same but their air volume (charge) won't be. In the same way, the larger plate of a larger cap needs more charge to produce the same voltage than a smaller plate.