Google has begun rolling out a series of algorithmic changes intended to more aggressively target hacked spam. These changes will disturb 5% of queries when it is fully rolled out. According to Google , it is trying to protect searchers and site owners. Legitimate sites can become dangerous if they fall in the hands of hackers.
A hacked site can be subjected to malpractices; google states that there will be a reductions seen in the amount of results shown. This is because they want to show only the most relevant results for particular queries.
With Google removing large amounts of hacked spam from search results, SERPs may begin to get smaller. An example is given in the announcement showing about a quarter of of a search results page being removed.