"The Maasai Mara is a big park"
Translation:Maasai Mara ni mbuga kubwa
Mbuga is not a word from the M-/Mi group, but from the N- group. In this group the adjective doesn't take a prefix. It only takes a prefix when the radical starts with a vowel (NY-), a D, G, or Z (N-), or a B, P, or V (M-).