Obsidian doesn’t show folders in the graph. You can manually shift select all the notes in a folder and drag and drop them into a designated note. This will automatically add links to all the files you had selected, thus showing this as a hub (of increased size) on the graph. However, you will need to update this note as you add more notes to the folder.
Another option is to use this plugin: Waypoint - Automatically generate content maps of your folders!
You may also want to look at this topic: Create a graph based on the folder structure
Finally, I would be remiss if I didn’t mention the Dataview plugin although it doesn’t affect the graph. Good luck!