Continuity mistake: When R2-D2 is hiding from the Sand People, he is standing in front of a rock. When he comes out, the rock disappears. 2011 Blu-ray and 2019 4K.
Star Wars (1977)
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Directed by: George Lucas
Starring: Harrison Ford, James Earl Jones, Peter Cushing, Mark Hamill, Alec Guinness, Carrie Fisher, Anthony Daniels, Kenny Baker, Peter Mayhew, David Prowse
Genres: Action, Adventure, Family, Fantasy, Sci-fi, Star Wars
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Trivia: The Imperial officer's uniforms were patterned after the uniforms of Nazi officers to add to their "villainous" image.
Question: How was Obi-Wan going to become more powerful after he was dead?
Answer: I agree with Darius Angel's comment. I also think Vader expected a certain sense of satisfaction after defeating his former master. In reality, though, he was still "owned" by the Emperor and was living with the consequences of his choices. Defeating Obi-wan did not change much for him. Obi-wan, however, gained the benefits that Darius Angel mentioned.
Answer: It is heavily implied that one who becomes a Force-Ghost achieves an untold level of power upon entering the state. Given that they have become a pure entity of the Force, it seems to back up the statement. Obi-Wan also becomes free to assist Luke in any case.
Darius Angel
In addition to this answer, I think Obi-wan also became powerful because he "let go." He did not feel a need to defeat Vader on this occasion - he was willing to surrender the fight and "move on" to a new state. Something that Vader might not currently understand.