There is nothing like 'Known' in dynamic data like demographics. In 2009 I think both Central and Western were very close - 3.4m - and now Central has overtaken Western by nearly a million votes. Admittedly most it going to Kiambu - that is sucking Nairobi excess population.
Eastern for long time use to be 2nd largest and populous - now Nyanza has overtaken them in population - as Meru, Embus and Kambas - see declining birth rate, and migration dynamics.
The dynamics of our tribal math.
If you must know in 1969 - Kalenjin were no 5 - with Kambasa No 4 - Luhyas no 3 - and Luos no 2
In 1989 - I think Kalenjin overtook Kambas to become no 4 - with Luhyas no 3 and Luos no 2 (or luhya also overtook Luos)
In 1999 - Tribal census was not released - but suspected Kalenjin had overtaken Luos.
In 2009 - It was Kambas - Luos - Kalenjin - Luhyas - Kikuyus.
My data seem to suggest - it now Kambas(?) - Luos(5.5m) - Luhyas(6.5M) - Kalenjin(7M) - Kikuyus(8M)
Not maybe, your maths is wrong. An increase of more than 10% to 15% in 10 years is not normal. even the Luhyas known for their love of babies cannot achieve that task.