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Out of interest, did you first notice the change in sizes with UK, European, or US brands?
I've only been wearing a bra for 17 years, so I obviously don't have your 25 years of fitting experience, but I've certainly never seen any change in sizing here in the UK during my bra-wearing lifetime. Maybe UK brands started to increase in size 15, 20 or more years ago? When I was a teenager I wore a 36C whereas I'm now a 30E. In hindsight the 36" bras were definitely 6 inches too big! No change in sizing, I just totally misunderstood what the numbers meant, and wore them anyway despite the poor fit.
Historically, it would make sense that sizing gradually became larger for the same reasons that all dress sizes become larger. As soon as the number representing the size ceases to refer to a specific measurement, it becomes a purely arbitrary number and the size inflation snowballs.
Case in point - women's dress sizes were originally based on "misses" sizes which were simply sized by age. In other words, a size 16 used to be an age 16. That's how far size inflation has come since the 1900s! You could say that this was appealing to women's vanity by encouraging them to think they can still fit into the same dress they wore at age 16. Bras seem to have kept the old women's sizing though, although they have become divorced from the measurements. The idea of adding four inches to the underbust seems to be an alternative to using the overbust measurement, which was the measure used for women's sizes before "misses" sizing took over.
7 months ago on How Bra Band Sizes Have Changed