  Mader proved that for every integer $t$ there is a smallest real number
$c(t)$ such that any graph with average degree at least $c(t)$ must contain a
$K_t$-minor. Fiorini, Joret, Theis and Wood conjectured that any graph with $n$
vertices and average degree at least $c(t)+\epsilon$ must contain a $K_t$-minor
consisting of at most $C(\epsilon,t)\log n$ vertices. Shapira and Sudakov
subsequently proved that such a graph contains a $K_t$-minor consisting of at
most $C(\epsilon,t)\log n \log\log n$ vertices. Here we build on their method
using graph expansion to remove the $\log\log n$ factor and prove the
conjecture.
  Mader also proved that for every integer $t$ there is a smallest real number
$s(t)$ such that any graph with average degree larger than $s(t)$ must contain
a $K_t$-topological minor. We prove that, for sufficiently large $t$, graphs
with average degree at least $(1+\epsilon)s(t)$ contain a $K_t$-topological
minor consisting of at most $C(\epsilon,t)\log n$ vertices. Finally, we show
that, for sufficiently large $t$, graphs with average degree at least
$(1+\epsilon)c(t)$ contain either a $K_t$-minor consisting of at most
$C(\epsilon,t)$ vertices or a $K_t$-topological minor consisting of at most
$C(\epsilon,t)\log n$ vertices.
