As cyclists have become taller over the years, bicycle frames have generally not grown as fast. As USAZorro points out, lots of vintage racing photos depict riders with very little seatpost showing.

There are two generally-accepted ways to measure frame size:
1) center of crankshaft, along the seat tube, to the center of the top tube;
2) center of crankshaft, along the seat tube, to the top of the top tube.

(1), popular in Europe, makes sense when one is figuring out frame geometry and measuring everything center-to-center.
(2), more popular in the US, seems the most practical to me, as it gives the most direct assessment of things like standover height (which can still vary because of BB height) and leg extension.

Having said all of that, I note that top tube lengthi is also extremely important in bicycle sizing, although one can compensate somewhat by changing the handlebar stem reach.