as those adopted by HA19, but HA19 defined these by the physical condition at $t=10$Gyr. We use the physical condition at each snapshot so that our definition in this paper really traces the current dense and diffuse phases. We choose the ages $t=0.3$, 1, and 3Gyr. These ages are basically chosen for comparison with A17. Although A17 also examined $t=10$Gyr snapshot, the snapshot at $t=3$Gyr represents the dust evolution at the oldest stage with a metallicity comparable to the present Milky Way; thus, discussions in this paper do not change even if we choose $t>3$Gyr instead of $t=3$Gyr.

In Fig. \[fig:size\], we show the grain size distributions at each age for the dense and diffuse gas particles separately with the median and 25th and 75th percentiles at each grain radius bin. The dust is dominated by the large grains at young ages ($t\lesssim 0.3$ Gyr) because stellar dust production is the major source of dust. Since star formation tends to be associated with dense regions in the galaxy, the peak in the grain size distribution is higher in the dense medium than in the diffuse medium. This leads to more efficient small-grain production by shattering in the dense gas particles 