The DB is just a repository for Virtual Center, it hold resource pools, clusters, folders, vm's, templates, etc., but just the entity names and relationships. Your VM's will still be attached to the ESX host itself, and you can either recover from a backup or just add your hosts to VC again and the VM's will be registered in VC and added to the DB repository again.
DB also holds performance/rollup data, that's where the real size comes from, we have around 450 VM's in our VC and our database is only about 20GB, must most of that is historical performance data.
