The images may not be illustrations, but whole pages. Or there may be hidden spacers.
Just start Sigil, Open the file and look in the left side under the book browser. There will be a folder for Styles. If there is a plus on it, click it and any stylesheets will appear. Double click on any one of them and it will appear in the main window.
Then you can look at it to see what is there.
Otherwise, click on any + that is with the Images folder and see what pops up.
Ditto with the fonts folder.
You don't have to know what to do about anything you find, but at least it will put you on the track of what might be making the file large.