did you do a lot of "theme changing"? Each time you select a different theme for a layout, your file get's a bit larger and this cannot be reversed.
Do you have a lot of graphics inserted onto your layout such as logos or as button icons? If these are hi res images, they can increase the size of your file substantially.
I had at one time explored lots of different themes for one or two layouts when I was trying different themes to see how they fit. Currently I use only two themes. But that exploration left unremovable debris behind? Ugh. That's probably the explanation.
No, I use very few graphics. I did see early on how a high-res image in a data file could affect file size, so stopped doing that!