Bearing in mind that there are nine and a half tribes in Eretz Yisrael and only two and a half in Eiver ha'Yarden, why do each of them contain the same number of Arei Miklat?
Rashi (citing Makos, 9b): This is because in Gil'ad (which was in Eiver ha'Yarden) there were a lot of murderers. 1
Ramban #1: Because Eiver ha'Yarden was an expanse of land equivalent in size to Eretz Cana'an. 2
Ramban #2: Of the additional forty-two towns, thirty-six were in Eretz Yisrael, and six in Ever ha'Yarden. All in all, each side had four towns per Shevet. 3
In which case the towns in both areas were spaced similarly, so the furthest that a murderer might need to run was the same in Eretz Yisrael and in Ever ha'Yarden. This determined the distance between the towns, and not the number of people who lived there. (EC) See Ramban.
Ramban: Most of Menasheh was in Eretz Yisrael (i.e. six of the eight families
QUESTIONS ON RASHI
Rashi writes that there were three Arei Miklat for two and a half Shevatim in Ever ha'Yarden, just like for nine and a half Shevatim in Eretz Yisrael, for there were many murderers in Gil'ad. Those murderers were Mezid. Ir Miklat is for Shogeg!
Da'as Zekenim, Hadar Zekenim: Since there were many Mezid murderers [and sometimes Beis Din cannot kill them, e.g. there were no witnesses], Hashem brought to there people who killed b'Shogeg without witnesses. A Shogeg murderer would accidentally fall on a Mezid murderer, in front of witnesses, and he would be exiled, and the Mezid murderer was killed; each got like he deserved, like it says in Makos (10b).
Ramban: Some murderers schemed to make it appear accidental, so extra cities were needed to absorb both the schemers and the true Shogeg murderers. Ritva