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May only looks better than Serena because she already has an adult figure despite only being 10 years old. ORAS May is not nearly as sexy, and actually is pretty generic looking and falls below other girls like Dawn, Hilda and Rosa.
Serena is dragged down by her hair style and clothing, but as fanart shows, she's stunning as an adult. Her mother is also stacked, so there's no reason to think she won't be after puberty, as is the case with Hilda and Dawn.