The figure is drawn from my studies from several years ago so I thought I'd revisit it.
Comparing the BOT stage plans to the JTB stage plans shows the difference pretty clearly - the end wall sticks out a lot more in the S2 version and the floor struts are in different positions in relation to the adjacent corridor.
The six 4' panels are clear to see in the S2 image which (including the five 1½" silver strips) adds up to a 24'7½" wall length.
Using the diagram from the Charlie X stage plan (which helpfully lists the wall panel widths) the same stretch of wall adds up to 19'7½" but the angled wall section does appear a little shorter in S1 for some reason, approx 2' difference. I suspect this is an error on the S2 sketch
That final wall panel also sticks out more beyond the outer angled panel in the S2 diagram, so just measuring the panel sections is not sufficient.
Measuring from the wall on the door end to the bottom of the angled strut yields a length of
S1 - 16'4½"
S2 - 19'6"
So in fact I had misremembered, because inside the room it is more like a 3' difference than a 4' difference.
Recently however I have grown to trust the stage plans less than I used to. They are fine as a general guide but not necessarily what was actually built for the series. Your question caused me to double check and I'm glad I did.
For onscreen evidence, I suggest comparing the final shot of
The Menagerie http://tos.trekcore.com/hd/albums/1x11hd/themenageriepart1hd610.jpg
To this scene from
Wolf In The Fold
http://tos.trekcore.com/hd/albums/2x14hd/wolfinthefoldhd1062.jpg
The rooms are the same on the right of the overhanding concentric beam. As for the panels on the left, look at the length(s) of the final whole panel on each picture and the shape of the angled beams - they are not the same.
There is at least a 2' difference in between the two shots.
So it's gone from 4' to 3' to 2'
Interesting
But unless the angles of the back wall changed from S1 to S2 (and I can't think why they would bother) then a 2' expansion seems to be the correct figure