  The anomalous large radii are exotic phenomena observed around the neutron
dripline. Around the neutron dripline, the weak binding of the last bound
neutron(s) causes the drastic increase of the radius, which is called neutron
halo structure. Although the nucleus $^{24}$O is located at the dripline of
Oxygen isotopes, the separation energies of one and two neutron(s) are 4.19 MeV
and 6.92 MeV, respectively. In spite of this enough binding, the enhancement of
the matter radius is observed. In this study, we microscopically describe the
structure change of $^{22}$O core in $^{24}$O and explain the observed large
radius based on the cluster model. Two degrees of freedom for the large radius;
the relative distances among four $\alpha$ clusters and size of each $\alpha$
cluster are examined, where Tohsaki interaction, which has finite range
three-body terms is employed. The nucleus $^{24}$O has the almost the same
amount of the clusterization compared with $^{22}$O, but the expansion of each
$\alpha$ cluster plays an important role. When two neutrons are added to
$^{22}$O at the center, the expansion of each $\alpha$ clusters is
energetically more favored than enhancing the clustering for reducing the
kinetic energy of the neutrons. The calculated rms matter radius of $^{22}$O
and $^{24}$O are 2.75 fm and 2.92 fm, respectively. Although these are slightly
smaller than the experimental values, the jump at $^{24}$O from $^{22}$O is
reproduced.
