I tried changing the Rect size to 0,0,0,0 and still have a 75MB mxb file.
Nice try. This adjusts how much of the data is displayed (in a particular image), but does not affect the data stored in the source table.
Here's how to reset everything to zero in SQL:
Let's assume the table is called [Image 1 Tiles] and has an image called [Image 1].
Open [Image 1 Tiles], and note what is shown in the tile column. For example, it might be <tile, 128x128, uint8x3>. We need to know the tile size (here 128x128) and its type (here uint8x3). If your image has different tile size or type then you will need to adjust the SQL below.
Open a Command window: View > New Command Window > SQL, or just Ctrl-~ (Ctrl-`).
UPDATE [Image 1 Tiles]
SET [Tile] = TileMakeNew(128, 128, VectorMakeX3(0, 0, 0))
Adjust the table name (NB the table, not the image). If the tile size is not 128x128, adjust the two 128 values to suit.
If the data type does not end in 'x3', change VectorMakeX3(0, 0, 0), e.g. to VectorMakeX4(0, 0, 0, 0) if it ends in 'x4'.
Run the code (F5). That may take a few minutes if the image is very large.
Now open the image, [Image 1]. You should see the warning bang at the right side of the tab. Hit F7 to bring up the Messages window and click Update Data to rebuild intermediate levels. Wait for that to finish (no progress window is normal here).
The image should now be blank.
Close all windows, delete any components you don't need, then export the project to .mxb. It should be tiny.
Can you also include the original metadata for the same image?