  Let $p$ be an odd prime and $L/K$ a $p$-adic Lie extension whose Galois group
is of the form $\mathbb{Z}_p^{d-1}\rtimes \mathbb{Z}_p$. Under certain
assumptions on the ramification of $p$ and the structure of an Iwasawa module
associated to $L$, we study the asymptotic behaviours of the size of the
$p$-primary part of the ideal class groups over certain finite subextensions
inside $L/K$. This generalizes the classical result of Iwasawa and Cuoco-Monsky
in the abelian case and gives a more precise formula than a recent result of
Perbet in the non-commutative case when $d=2$.
