In addition to the good information provided already, I'll add that some builders that provided a full range of seat tube sizes used a much more limited range of top tube sizes. It almost seemed as if they used the same top tube length regardless of the size of the bike. Some older Raleighs show this characteristic, if I recall correctly. Smaller riders without long torsos would not find a good fit. This tendency seems to have become less prevalent by the 80's as proportional geometry was widely adopted. So it depends on the era of the bike to some extent whether or not this is a potential issue.