Why did Aamir Khan and Kangana Ranaut cut their co-stars scenes in movies?

There are leading celebs who do exercise some creative liberty during the making of a film. Aamir Khan is known for being a perfectionist and this reflects in his films as well. He tries to ensure that everything in his movie flows in a certain manner. But this has also led to some of his co-stars' scenes being chopped from the film. During the making of Dhoom 3, Aamir got a solo song of Katrina Kaif removed. While Katrina's role wasn't very big on screen time, there was actually more of her character. An additional song was meant to explain Katrina's history and arc which would've allowed the audience to know her better.

While Aamir is at least established and has a sense of direction, another celebrity who has gotten involved with movie making is Kangana Ranaut. In fact, she's notorious for upsetting the crew with her directorial vision. She made a lot of changes to Manikarnika: The Queen of Jhansi, and even cut a lot of Sonu Sood's scenes which eventually resulted with him leaving the project.

Why do some actors chop off their co-stars' scenes?