"Snippet = A short description of or excerpt from a website which appears in Google search results. Snippets are created automatically based on the site's content or the content of pages that link to the site." (Google Search glossary)
In some cases, Google shows longer snippets for search results if the pages provide simple answers for your query. Google previously tested a sidebar that allowed you to increase the size of the snippets and highlight extracted facts like dates and locations, but the tests probably showed that people didn't use the advanced options and that it's a better idea to automatically increase the length of a snippet if there's a high density of information related to your query.
Search Engine Roundtable suspects that "the longer your search query, the longer the snippet might get", but this is not always the case. Google tries to be smart and not overwhelm users with irrelevant information, that's why snippets are usually concise.
Here's how Larry Page described snippets in 1998, when the feature has been launched: "We recently added some new features to Google. The most significant is a summary for each result that highlights where your query matched. This makes for a much more informative summary than most search engines provide. You can actually see where your query matched without having to download each page."
Find facts from snippets
From search results to content creation