Well, it could be just a random physical characteristic given for comical effect. Maybe the author wanted to create a contrast between Oliver Wood's more solid, perhaps muscular build compared to Percy Potter. It might also be used as a way to emphasize Oliver's presence or strength in a humorous way.
In Potter fanfic, Oliver Wood is often depicted as a very dedicated Quidditch player. When it comes to 'greater weight' in relation to him, it could refer to the heavy responsibilities he has as the Gryffindor Quidditch captain. Percy, on the other hand, is known for his strict adherence to rules. In fanfics, these three elements can be combined in various ways. For example, a fanfic might explore how Oliver's leadership in Quidditch and the 'greater weight' of his role affects Percy's view of Gryffindor house's reputation, especially since Percy is so concerned with the house's standing in the school.
You can find Oliver Wood fanfic on websites like Fanfiction.net. It has a large collection of fan - created stories, and you can simply search for 'Oliver Wood' in the search bar. Another good place is Archive of Our Own (AO3). It has a well - organized tagging system, so you can easily filter out the Oliver Wood fanfics from the vast amount of content there.
Another aspect could be related to his age and growth pattern. If in the fanfic's universe, Oliver has gone through a growth spurt earlier than Harry and has filled out more. His body could have developed more mass in terms of both muscle and some extra fat as he entered adulthood or a more mature stage of growth before Harry did.
One possible plot could be that Oliver, being the Quidditch enthusiast, asks Hermione for help with strategizing for an important match. Hermione uses her intelligence and knowledge of spells to come up with unique plays. Along the way, they develop a mutual respect and friendship.
Maybe it's because Oliver Wood is more muscular. He's a Quidditch player and all that physical training might have built up his muscle mass more than Percy, who is more into academic and administrative work. So the difference in their body composition could lead to Oliver being much heavier.
One possible fanfic could be where Oliver Wood gets injured during a particularly rough Quidditch match against Slytherin. Maybe he's hit by a Bludger that was jinxed by the opposing team. His injury could lead to a young Gryffindor player having to step up and take on the role of the Keeper, facing great pressure and self - doubt as they try to fill Wood's shoes. This could also cause a rift in the team as some blame the new Keeper for not being as good as Wood right away.
I'm not sure specifically which fanfic this refers to. But generally, if in a fanfic Oliver Wood is heavier than Percy, it could be shown through physical descriptions. For example, when they are standing side by side, the author might describe how Oliver's frame is broader and more solid - perhaps he fills out his Quidditch robes more fully compared to Percy. Or when they are participating in some physical activity like climbing stairs, Oliver might be shown to be a bit slower or more out - of - breath due to his extra weight.
You can find 'Harry Potter and Oliver Wood fanfic' on fanfiction.net. It has a large collection of fan - created stories related to the Harry Potter universe, including many focused on the relationship between Harry and Oliver. Another place is Archive of Our Own (AO3). It's known for its wide range of fanfiction, and you're likely to come across some great stories about them there.
In Potter fanfic, Oliver Wood's 'greater weight' could be his leadership role in Quidditch. This might be related to Percy as Percy is someone who respects authority. So, Percy could either support Oliver in his role, like helping with administrative tasks related to the Quidditch team. Or, if Oliver's actions seem to go against Percy's sense of order, there could be conflict.