Lots of factors to consider here.
The last decade has seen frame manufactures redesign the geometry of frames. The compact frame with a sloping top tube is not a performance enhancement. It's actually a way for frame manufacturs to make no more than 4 frame sizes for all size riders. It is much cheaper for a manufacturer to produce 4 frame sizes than a full range of frames in 1 or 2 centimetre increments ranging from 47cm to 62cm. For most people, riding a compact frame is a compromise in frame geometry, even though they probably don't realise it.
Compact frames have greater clearance over the top tube due to the shortened seat tube, which allows shorter riders to fit on a bigger frame without it being apparent to them that the frame is too big. Big riders have a whopping amount of seatpost showing. The mrketers will have us believe that a smaller frame is stiffer and lighter. This is true, but the longer (heavier) seatpost, which is not part of a triangle, is free to flex all over the place.
This new frame geometry has resulted in frames being measured in all sorts of new and interesting ways. Whereas historically, a frame was measured by the length of the seat tube, from centre-centre, they are now almost always measured by the length of the top tube, or, sometimes, the effective top tube length, which is an approximation of what a traditional frame's top tube length should be.
Now the truth is, that top tube lengths on traditional frames are much less variable than seat tube lengths on traditional frames, so, statistically speaking, a person who is an outlier when sized using the seat tube measurement, is likely to appear much closer to the mean frame size when sized using a top tube measurement. These days though, the top tube length is a function of its slope, and many newer frames also have a curved top tube, so the measurement cannot be compared from one brand to another. See the trickery going on here? I'm certain that frame builders these days are deliberately trying to make it harder for people to know what frame size they fit, since it can vary from brand to brand, and even model to model. Once you find a bike you like, it is much harder to compare it with other brands.
Next, I would say that overall, bikes that are from the 50s and earlier, have a tendency to have noticably different geometry to later bikes. They have longer wheelbases, flatter headtubes and slower steering. I put this down to the fact that riders spent as much time riding on unpaved roads and trails as riding on paved roads. The motorcar was a luxury back then, and almost no-one had them, so there wasn't the same road infrastructure we have today. Anyway, the point I am making, is the more relaxed angles (including the seat tube angle) meant that the seat tube needed to be longer to raise the rider to the same vertical height. You can confirm what I am talking about if you can find one of those old bikes with the "Major Taylor" style seatpost, where the saddle can be slid forward along the flat section of the seatpost to bring the rider back to a more upright riding position. In those days, setback seatposts were unheard of, and everyone had "set forward" seatposts (and by corrollary, longer seattubes), which equals bigger frames, even though the rider is in pretty much the same riding position.
The next thing to consider is wheel size. These days 700C is the norm, but bikes in the 50s ridden by sports clubmen were more often than not built with 26 x 1 and 1/4 inch, which if memory serves me right is 590mm in ETRDO sizing, and quite a bit smaller than 622mm of 700C rims. This may not make much difference to frame angles, but is likely to increase the standover clerance. Incedentally, when traditional bike frames were the only option the bike shops all told me that I should have about an inch clearance above the top tube when standing in relaxed fashion over a road bike.
