I have a couple of excel pricing models that have this issue.
I’m rocking Excel 2016 MSO 32-bit, via Windows 7 Professional.
When I makes some changes to certain sheets, it appears to be when I change the colour of a cell (but not every time though), the file size immediately jumps from 300kb to almost 3MB.
The big hint that this has occurred is that the row header bar becomes much wider than normal after going more than one screen down the sheet.
I have isolated the ‘bloated’ sheet in question and stripped absolutely everything from the sheet.
I’ve deleted all of the rows and columns right the way to the end, and cleared all conditional formatting, and removed all define name ranged, and links to macros. To every conceivable extent, this is now a file with one sheet/tab, that is absolutely blank in all respects…except it is still carry a weighty size of 2.6MB.
I can post/send this file to anyone who is lovely enough to take a look for me.