because of higher rates of grain–grain collisions. In the dense ISM, [ the small grains produced by shattering are further processed by accretion, which forms a bump (second peak) at $a\sim 0.01~\micron$. The bump is formed at smaller radii than the main peak because the large surface-to-volume ratios of small grains lead to efficient accretion [@2012MNRAS.424L..34K].]{}

As shown in the middle panel of Fig. \[fig:size\] at $t=1$ Gyr, the grain abundance becomes larger at all grain radii than at $t=0.3$ Gyr, because of further dust enrichment. The increase at small grain radii is more significant than that at large radii because of further dust processing by shattering and accretion. In particular, dust and metal abundances are large enough for efficient accretion at this stage. Therefore, the bump created by accretion around $a\sim 0.01~\micron$ is commonly observed in the grain size distributions. The dense gas particles have more large grains than the diffuse ones for two reasons: one is the same as above (we find more dense gas particles in actively star-forming regions), and the other is more efficient coagulation in the dense ISM. Indeed, we observe that coagulation depletes the smallest grains in the dense ISM.

In the bottom panel 