I agree with everything elvin said and would also ad that while Harry is naturally skilled as a flyer he also had a world class broom as a 13 year old, that's a huge advantage, furthermore it's not too much of a stretch to think that Fred and George are A). close to being world class beaters. B). being "body builders." Or at least body builders compared to Harry. To answer A) I think they were described as great beaters in canon who were great before Puddlemore. Not to mention they've been training every day for a few years, not to mention they spent every summer playing, while Harry only had quidditch practices. B) They're professional athletes who spend they're days smacking a very heavy ball using a very heavy bat.