  We consider categories over a field $k$ in order to prove that smash
extensions and Galois coverings with respect to a finite group coincide up to
Morita equivalence of $k$-categories. For this purpose we describe processes
providing Morita equivalences called contraction and expansion. We prove that
composition of these processes provides any Morita equivalence, a result which
is related with the karoubianisation (or idempotent completion) and
additivisation of a $k$-category.
