In planet formation theory, sub-micrometer-sized dust grains first grow into kilometer-sized planetesimals. The dust aggregates formed during this process are theoretically expected to have a very low density. However, astronomical observations by ALMA suggest that dust aggregates in protoplanetary disks are sub-millimeter-sized and have a high density. This means that some compaction processes are necessary. In this work, we focused on comets as remnants of planetesimals and investigated the dust growth process by using the material strengths of dust aggregates. First, we calculated the compressive strength of dust aggregates by using numerical simulations that take into account the contact mechanics of dust particles. Then, we calculated the bulk density and size of dust aggregates, which are determined by the balance between compressive strength and self-gravity, and compared them with those of comets. As a result, our results indicate that comets can be explained by dust aggregates that consist of more than 1-µm-radius particles. We also found that the self-gravity compression of planetesimals larger than 100 km in size is necessary for the formation of high-density dust aggregates observed by ALMA.