Companies change as they get bigger because of increasing risk. The company has more value and making big gambles risky ventures with a large company starts to become irresponsible.
But yes, in many ways it is less risky for a big company to try to acquire successful innovative small companies rather than trying to do the innovation in-house. But they criticized for that, too.
1. purchase for innovation.
2. market control.
I think the latter is what people squawk about the most. Employees of the acquired will always find it uncomfortable. Small companies often have limited process where larger companies generally need it to function. That being said I don't think larger companies cull/assess the impact/efficiency of their processes on a regular basis.