  We investigated structures of low-lying $0s$-orbit $\Lambda$ states in
$p$-shell $\Lambda$ hypernuclei ($^A_\Lambda Z$) by applying microscopic
cluster models for nuclear structure and a single-channel folding potential
model for a $\Lambda$ particle. For $A>10$ systems, the size reduction of core
nuclei is small, and the core polarization effect is regarded as a higher-order
perturbation in the $\Lambda$ binding. The present calculation qualitatively
describes the systematic trend of experimental data for excitation energy
change from $^{A-1} Z$ to $^A_\Lambda Z$, in $A>10$ systems. The energy change
shows a clear correlation with the nuclear size difference between the ground
and excited states. In $^7_\Lambda \textrm{Li}$ and $^9_\Lambda \textrm{Be}$,
the significant shrinkage of cluster structures occurs consistently with the
prediction of other calculations.
