  Performing $n$ steps of $\beta$-reduction to a given term in the
$\lambda$-calculus can lead to an increase in the size of the resulting term
that is exponential in $n$. The same is true for the possible depth increase of
terms along a $\beta$-reduction sequence. We explain that the situation is
different for the leftmost-outermost strategy for $\beta$-reduction: while
exponential size increase is still possible, depth increase is bounded linearly
in the number of steps. For every $\lambda$-term $M$ with depth $d$, in every
step of a leftmost-outermost $\beta$-reduction rewrite sequence starting from
$M$ the term depth increases by at most $d$. Hence the depth of the $n$-th
reduct of $M$ in such a rewrite sequence is bounded by $d\cdot (n+1)$.
  We prove the lifting of this result to $\lambda$-term representations as
orthogonal first-order term rewriting systems, which can be obtained by the
lambda-lifting transformation. For the transfer to lambda-calculus, we rely on
correspondence statements via lambda-lifting. We argue that the
linear-depth-increase property can be a stepping stone for an alternative proof
of, and so can shed new light on, a result by Accattoli and Dal Lago (2015)
that states: leftmost-outermost $\beta$-reduction rewrite sequences of length
$n$ in the lambda-calculus can be implemented on a reasonable machine with an
overhead that is polynomial in $n$ and the size of the initial term.
