The Microsoft case was different--Microsoft's sin was using their position to force the distribution of IE in an effort to destroy paid competition. I don't see anything like that from Google.
They do some of that. If you're a start-up that does specialist searches, say a restaurant review site, Google will simply replicate the information from your site in a search summary, without paying you a dime. Several Google services compete with smaller companies, who find that their content appears on their rivals site, and when searching for their content, their sites appear below Google's in their searches. This quite effectively acts to restrain their business.
A breakup would prevent this. By stripping the search engine business out from the rest, Google would then need to account for information sharing between the two, and such measures would fall under existing anti-trust and competition legislation.
Moreover a breakup could be a prelude to a more comprehensive measure, forcing Google to locate its servers in Europe, thus making it much harder for Google to be used by US security organisations for industrial espionage in Europe, and to spy on European governments. The main obstacle to that is that Google is a large business with most of it's interests in the US, but if the search engine business is split off, then it becomes a much more achievable goal.