OK, here are my findings.
1/ There are no images in the file - fair enough.
2/ Your menu arrangement is complex and this is leading to the size of the stored data for your page Note this size is not published but is all the stack settings etc etc that has to be part of your project.
To be clear, when I say complex, your menu is built from seventy (70) separate stacks. I may be slightly off there, counting so many is arduous.
There are many pages that don’t have 70 stacks on them in total so just for the menu this is quite large. This is accounting for 1.6MB on average of hashed data that the Stacks plugin has to store in order to remember your choices and settings.
To put this in context - 1600kB divided by 70 is 22.86kB per stack. Anyone think this is particularly fat? Remember we are talking about project size and not published code which is considerably less.
Again, almost none of this is published so the final page is OK - it is just the project (and associated edit mode performance).
On the other hand there are some words of caution regarding the mega menu stack as follows:
It loads 16 separate fonts in edit mode with absolutely no option for the user to prevent one or any of them loading. This can cause page jumps as the links are followed - something which people often attribute to stacks where it is optional to load each font and the user has self inflicted it (like Paragraph Pro or Font Pro). In this case an enormous amounts of fonts are always loaded in edit mode which is agreed bad practice.
You are building a very large site with a lot of pages. Having a manually curated menu for so many pages that by necessity uses such a large number of stacks in its construction will always cause your project file to be quite large.
70 stacks for the menu before you even add page content says to me that you should either be doing it in a more simple way or if that sort of navigation is a prerequisite then RW is not the appropriate tool for the job.