Try going for the regular parts of life for content.
For example: Say the plot of a story is that a female university student is on a scholarship for running but gets injured leading to weight gain. There are plenty of details that can be filled out for drama and content. Life is a constant and complex thing after all.
Roommates or housemates are common and give opportunities for interaction, same with friends, classmates, rivals, family or the coach. These characters might at first bring gifts of food or offer assistance at first allowing the weight gain to start. But later could change to comments and concerns of about the main characters weight gain. Or supportive and wanting the main character to be happy if this new lifestyle does that for them. Which can lead to arguments between the main character and them or the supportive characters versus the those against weight gain.
Eating is important for life and gaining so how does the main character do so:
-Who are they eating with? By themself, with friends or classmates who, as stated above, can talk with the main character. This talk could be reminders to take it easy to heal, suggestions for things to do to pass the time while healing that the main character now has time to try, comments about weight or lifestyle changes.
-What are they eating? Are they sticking to a meal or diet plan for their athletics that is no longer filling them up because of snacking? Or they feel like treating themself to desert for a change or new offer or encourage larger food portions.
-Where are they eating? Relates to Who but are they eating alone in their room, at a cafeteria or food court at the university, or somewhere else. If somewhere else maybe they see something that interests them in a nearby store like more comfortable, and baggy or stretchy, clothes, video games, TV series and movies or even books or comics / manga.
-When are they eating? If the main character usually eats with friends on the track team then being injured means they are no longer going to practice so their daily schedule is is different. This could lead the main character to eating at a different time and meeting new people who are supportive of being heavier. This could also lead to eating more as perhaps the main character would snack during practice and burn it off but now they are snacking and not exercising.
Chores such as laundry or getting rid of garbage can lead to interactions with roommates or housemates. Classwork and projects as well though with friends and classmates. Does the main character have a job or end up getting one to support their growing eating habits. Have the main character join a club for a TV series or movies, gaming of some type or other form of entertainment. Also don't forget about events such as concerts, expos, and more that the main character could go to.