In may cases, the entire site structure can be contained in a single site collection, andn indeed this is convenient because the built-in navigation is based on the sites and page within the collection. In addition, content queries, content type, storage quotas, and numorous other sharepoint capacitites are scoped at a site collection, so there is a tendency to design large site collections to make them work over a large set of content. a site collection is alwyas stored in a single sharepoint content dtabase, although a content database con contain many site collections. Many sharepoint administrators limit site collections to 50-200g , and place large connections in their own, dedicated content database.
sharepoint's build-in groups are scoped at site-collection level, so if seperate sets of groups are desired for administrative control, separate site collections will be necessary.
Some sharepoint features are scoped at the site collection level, if these features are desired in some areas but not others, then the areas need to be in different site collections.
Anonymous access is coped at the site collection level, so if part of a web site is to be open to anonymous users, where another forces a login, these sections should be in separate site collections.
a common pitfall is to build a solution with one giant site collections and find out months or years after that the datbase has become too large to restore from backup within the service=level agreement. If a site contain large items such as video, then consider putting them in a separate site collections and linking to them to divide the storage.