The Minmatar were found in 3 planets, when the Empire invaded. (apparently though, according to Theodicy, there were other planets by the time of vak-atioth)
At the end of the Rebellion, 66% of the Minmatar population were freed, and were then living in what is now Republic space (a whole lot more planets). 33% remained in the Empire
In the modern age, 33% of the Minmatar are still in the Empire, but only 25% of the Minmatar still live in the Republic, with 20% in the Federation, and the rest, distributed across the rest of the cluster.
There are things that mention "population boom", in missions and such, but, how does that explain the massive shift in numbers ?
30% of the Federation population is Minmatar, but they've only been immigrating there for 130 years. Seems a big wave of immigrants and/or enormous numbers of children, and/or young mothers, having children at 20, rather than say 40 as may be the case for Gallente people.
Did Minmatar people emigrate en masse from the Republic ?
https://wiki.eveonline.com/en/wiki/Demographics_of_the_Amarr_Empirehttps://wiki.eveonline.com/en/wiki/Minmatar_Republichttps://wiki.eveonline.com/en/wiki/Demographics_of_the_Gallente_Federation#Non-member_races