Originally Posted by dddd

What I was saying is that if you are measuring your frame's forward reach (from the bb shell forward), that whatever length of top tube that you have, a slacker seat tube angle will pull the entire top tube (and both of it's endpoints) rearward, relative to the bb.
The result is that the big bike now will feel much smaller while riding, equivalent in some cases to several cm of toptube of stem extension length!

A 24" (61cm) Schwinn Varsity frame thus feels about like a 55cm(!) frame while riding, especially since the similarly-slack headtube angle will not provide neutral steering if the stem extension exceeds 9cm, unless proportional increases in handlebar width are also made, which is limited.

On my 24" Varsity (the silver/gray one pictured above), I thus had to shorten the ends of the handlebar, for knee clearance, even after replacing the original 7.5cm steel stem with a 9cm stem. And I'm only 5'9".

The modern specification of frame sizing, using the new terms "stack" and "reach" are used remove the complicating effect of bottom bracket height and seattube angle, respectively, when describing the principle frame fit parameters of the frame's height and forward reach.
This will only be useful however when comparing frames that are both described using these terms, or perhaps when sizing a new rider to a new bicycle.