Hi there,
every element you add to a page, whether that be a paragraph, a heading, an image, a container, a column etc. will output HTML. Each of those are counted in your DOM Size.
Unless you have plugins that are creating bloated HTML that can be replaced, then the only way to reduce that is to remove content from the page, or use some kind of ajax loading. Otherwise just ignore the Diagnostic ( its not an error ), it only becomes an issue if you have a lot of styles or interactive scripts on the page