Various mfgs. had different ideas, take Gios, top tubes way to short for me at my size. Nishiki also had short top tubes for a given size, for comfort basically, except the smallest that were too long to avoid toe clip overlap). Do not forget that seat tube angle makes a big difference. My first track bike, a Bob Jackson had a 22-3/4" top tube! but I used a 125mm stem on it as to put my seat in the correct position I had to bypass the shallow seat tube angle. While top tube length is a guide, seat tube angle can mean even more regarding on where the builder expected you to sit.