Gain land by pressing claims of others?

This seems to be a question for myth busters. The point is that the claimant that I press will become my vassal. Everywhere I search for an answer people are answering differently.

• The claimant should not gain a higher title then yours.

To me this means I can vassal an entire kingdom if I am emperor or a county if I'm a duke by just inviting someone with a claim to my court and press his claim in a war. He will become my vassal and my land spreads

• You should land the claimant a higher title then he can gain

In this case you can only do this as a king (or perhaps petty king?) since as a duke I have to give away my title of duke in order to gain a county as a vassal. Likewise, this seems unwanted if I'm an emperor, as the emperor of 3 kingdoms I would not want to give away 1 kingdom in order to gain a duchy.

Even if I could gain a kingdom I would end up with a vassal holding as much kingdoms as I do making him extremely powerful.