Export Assets giving large extra white space, inconsistently: top bottom left right. Never had this problem before. Happens no matter what format is chosen.
The extra space is there because of the clipping mask used for the wood texture. Adobe Illustrator does not trim to the clipping mask when using the Export Assets panel. This is a bug in Illustrator and has been reported.
You have a couple of work arounds:
1) Move the graphic with a clipping mask to the Libraries CC panel. Then move the artwork from the libraries panel back to the artboard. Delete the original graphic with the clipping mask or hide that object. Move the artwork from the artboard to the export asset panel.
I do not recommend using the following but they still work.
2) Use multiple artboards for each individual asset you want to export. You would then have to export multiple times for the various sizes you want for your assets.
3) Create a new Illustrator file for only the graphics that use a clipping mask. Then place that Illustrator file in your other Illustrator file that you will be exporting the assets from.