Maybe in the fanfic, Bucky has been through some sort of experiment that affected his metabolism and made him weigh less. Steve, being the protective friend, starts to train with Bucky to build up his strength again. They go through a series of challenges together, both physical and emotional, as they try to get Bucky back to his normal self.
In this fanfic, it could be that Bucky has been on a long mission where resources were scarce, leading to him losing weight. Steve, who has just returned from his own mission, is shocked at Bucky's appearance. He then decides to take Bucky on a relaxing retreat where they can fish, hike, and get Bucky to gain back the weight in a healthy and stress - free environment.
The fanfic could explore how Bucky feels about Steve's concern. Bucky might be a bit prideful and not want Steve to fuss over him. But Steve's worry comes from a place of deep friendship. As Bucky weighs less, it could create a shift in their dynamic, with Steve taking on more of a caretaker role, which could lead to some interesting conversations and moments of tension between them. For example, Bucky might try to prove he's still strong despite his lower weight, and Steve might have to learn when to back off and let Bucky have his independence.
On the other hand, Steve might be more understanding than expected. He knows about Bucky's past traumas and could suspect that something else is at play. So, instead of immediately resisting, he might play along for a while, waiting for the right moment to figure out what's really going on and how to help Bucky.
In some fanfics, it might be justified in a complex alternate universe scenario. Maybe Bucky is mind - controlled in a way that's extremely difficult to break, and he's forced to act against Steve. However, this goes against the deep - seated friendship between them in the main canon.
You can find 'bucky and steve fanfic' on Archive of Our Own (AO3). It has a large collection of fanfictions related to various fandoms, including those about Bucky and Steve. Fanfiction.net is also a good place. There are many talented writers who post their works there.
One possible plot development could be that Steve realizes his strength has accidentally put Bucky in a dangerous situation. He quickly tries to lift his weight off Bucky and helps him up. Then, they might have an emotional conversation about Steve's powers and how they need to be more careful around each other.
One possible plot could be a humorous situation where Steve is carrying Bucky in a rescue mission or just during a training exercise and he keeps complaining that Bucky is too heavy. Maybe it leads to some comical banter between them, like Bucky making jokes about Steve's weakling days in the past.
Perhaps in the fanfic, the weight difference is symbolic. Oliver Wood's greater weight could represent his groundedness, his connection to the more physical aspects of the wizarding world like Quidditch. Percy Potter, if he's more into academic or bureaucratic matters in the story, might be presented as lighter in a sense of being less 'tethered' to the physical world. It could be a creative device to show how different their worlds and focuses are within the narrative of the fanfic.
There are quite a few. 'Bonds of Friendship' is a popular one. It focuses on how Bucky's recovery impacts his relationship with Steve. 'Together Again' is also good. It has a lot of emotional moments between the two as they try to find their place in a world that has changed so much. Then there's 'The Unbreakable Bond' which really emphasizes the strength of their connection.
In some fanfics, a motive could be a deep - seated jealousy. Suppose Steve gets all the glory and recognition while Bucky feels left in the shadows. This could build up resentment over time and lead to a situation where Bucky snaps and kills Steve. However, this is a very different take from their normal relationship in the official stories and really plays with the emotions and expectations of the readers.
A good Bucky x Steve fanfic often has well - developed characters. It should stay true to their personalities from the MCU or comics. For example, Bucky's struggle with his past and Steve's unwavering loyalty.