are all frame bags made to measure?
if so, why,
a bag for say a xl steel framed 29r would would surely fit a on one/swift/cotic/niner.......
a 21'' frame frome these builders measures the same.
and would this not follow suit down through the frame sizes?

just curious.

A lot would fit I think but I suppose you would end up with I'll fitting corners and what not. Different frame angles and stand overs etc.
The Revelate tangle bags fit pretty well cos there's space at the bottom for frame differences.

I think bags often will fit between similar sized frames. The biggest headache I've found is the location of cable stops, front mech's, etc meaning straps are often in the wrong place ... I can't use the same frame bag on both of my Inbred 29ers although they're the same size due to the above. I can however use one on my Haro SS without issue.