"The book is heavy"
Translation:Kitabu ni kizito
In informal conversations, you will often hear people switch the words but they always (if not often) refer to power or gravity of something.
Yule mzee ni mtu mkubwa/mzito (Than man/oldman is powerful/influential) (lit. That man is a big/heavy person)
Hii ishu ni kubwa/nzito (This case is serious) (lit. This case is huge/big/heavy)