Whybis is not "mti mrefu"?
I believe "mrefu" means "tall/long". "Big" could refer to the size of the trunk, how wide branches spread, etc. Not necessarily the height.
Why does it matter which order it goes in?
It matters because it would be unright to say mkubwa mti
So you would put mti mkubwa
Glad you were able to answer your own question ...
Indeed, so we can make the analogy in English it would be wrong to say "the tree big" the correct English order being "the big tree".