Indeed, it would seem that in going from all-drums to all-discs, you should actually need, if anything, a SMALLER master cylinder rather than a larger one:
http://www.aa1car.com/library/brake_calipers.htmThe apparent improvement observed when changing to a larger master cylinder may be just coincidental, in that by disturbing the whole system and re-filling/re-bleeding, the problem with the original set-up (most likely air still trapped somewhere) has been cured.
Normally you wouldn't disturb the original master cylinder unless you planned to change it. But did anyone ever actually remove it fully, as if to replace, but then re-fit the SAME master cylinder? According to this theory, that should give the same improved performance as is attributed to the replacement larger cylinder.