Central [Province]; Rift Valley [Province]; etc are meaningless concepts in the Constitution of Kenya - they don't exist! So it is improper to use them as a basis of any comparison.
Also: the subdivisions is NOT based on number of registered voters! It is on the total population according to the latest available data from a census.
The Constitution divides the country into Counties. Forex ample, Kikuyu constituency is in Kiambu while Mathira is in Nyeri...
Nevertheless; when I looked at the names and populations listed in the press adverts, it appeared to me that all had the constituencies had almost equal populations - except those in Nairobi which had more.
But I must admit that this was just a casual perusal. Can @Burning Spear give the populations of the three new constituencies that were created from Kikuyu and compare it to Mathira.
Nothing is real unless it can be named; nothing has value unless it can be sold; money is worthless unless you spend it.