Does size really matter?

Many different suppliers have two sizings: Under 30 and never gave birth vaginally and then, over 30 and/or vaginally given birth. These companies state the 3mm difference in size is crucial, but don't elaborate why. My question stems from the fact I've a 'one size fits all' cup, Femmecup. This cup is at its widest point, 45mm. The under 30 cups I've seen on these sites are 43mm and 46mm for the overs and mothers, now if 3mm is crucial... I'm worried my cup, at 45mm is maybe too big? I'm 22, and childless. What would be the effects of wearing an ill fitting cup? What's your thoughts please dolls :)

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