What is different between using "I have expected" or "I expected" instead of "I would've expected"? It seems all same to me.
You are correct in assuming that using "would've/would have expected" and "expected/have expected/" could have both been used here with no change to the sentence meaning. The difference is that "would have" indicates hypothetical expectations (although none were made), whereas just using "have expected/expected" indicates a speculation was actually made in the past. Thus, you can understand Hagrid as saying "you're a bit more along than I would've expected (had I made any expectations)," meaning that Dudley is fatter than he expected, had he had any expectations. Ex: You're younger than I would have expected (if I had any expectations). (Hypothetical expectations, had they been made) Ex: You're younger than I expected. (Actual speculation made in the past) It's quite tricky to differentiate the two, but you can use this simple trick: if actual expectations were made, use the simple past tense ("expected.") If they weren't, use "would have expected."