C# Garbage Collection Issue

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C#

I have an application I'm working on that opens files into a list view and lets the user modify them and other such tasks, the files are stored as archives and comprised of multiple other files. So each of these files is turned into an object with refferences to the files (also turned into objects) that make up the archive. The problem I'm having is that when the user removes the file from the interface the garbage collector doesn't seem to release all the memory the file was originally using. This is probably confusing (I know I'm confused after reading it) so here's basically what I have:

class mainFileItem:ListViewItem
{
    private mainObject main;
    //Bunch of other stuff
    
    public void Delete()
    {
        this.Remove();
        this.main.Dispose();
    }
}

class mainObject:IDisposable
{
    List<subObject> subObjects;
    //Other stuff
    public void Dispose()
    {
        foreach(subObject sub in this.subObjects)
        {
            sub.Dispose();
        }
    }
}

class subObject:Idisposable
{
    private List<Resource> resources;
    private Object someObject;
    private sting someString;

    public void Dispose()
    {
        this.someObject.Dispose();
        this.someString = null;
        foreach(Resource something in resources)
        {
            something.Dispose();
        }
    }
}

class Resource:IDisposable
{
    byte[] data; //Very large, contains the resource file as raw data
    
    public void Dispose()
    {
        this.data = null;
    }
}

This is a close approximation to what my code looks like. What I'm finding is that when I call "Delete()" in "mainFileItem" not all of the memory is released (when I load the file originally memory use increases by ~20K but after "Delete()" memory use only drops ~2k). My guess is that when I call "Dispose()" in Resource "data" is not released correctly (but that's just my guess).

Additional Notes:
Whenever I call "Dispose()" in any class I set every member variable to null or call "Dispose()" on it.