I think it is about the size and complexity of the integration but first you have to answer how much you are integrating? Is this full or more minimal integration, which is a question regardless of the size of the company. If minimal integration is required, it might not matter as much the size of the company. In many ways you are doing the same activities regardless of the size, so the question is, would you rather do those activities many times over with less risk or do them once for a bigger ROI.