Becoming a very successful, very powerful company very quickly is bound to bring growing pains. However, competition concerns are not growing pains. No company is investigated on antitrust grounds merely because it is large. There has to be more.
In Google’s case, the complaints that are currently being investigated by a number of European competition authorities (as well as being litigated before US and European courts) concern whether Google has used its dominant market position in search and search advertising in ways that are anti-competitive.