Construct keeps the tiles past the edge of the tilemap to preserve them essentially for undo purposes - if you slip and accidentally resize a tilemap smaller than you want, that definitely shouldn't erase all your work. So you can resize a tilemap smaller, then resize it bigger again, and see all your old tiles. I think everyone would agree that's useful.
I do however agree that Construct shouldn't save all that extra data to the project. I think that is an oversight, and at that point the data should be truncated, much like nobody expects undo to still be there after opening a project, in order to save memory. That's straightforward to do and is implemented for the next beta. FWIW, that could have been filed as a bug report, as I don't think there is any case to preserve that data through saving and opening a project.
I also went further and made it so the editor will only fully load all tile data for tilemaps in open Layout Views. When you close a Layout View it will then save the tile data in a more compact form until it is needed again, which should also mean the editor scales much better with large projects using tilemaps extensively, as it will only need to have full tile data loaded for your currently open Layout View tabs.