I never use the "export selection" option in Illustrator for that reason. When exporting, illustrator seems to export the entire area containing all content within the file, so that means unseen paths, points, text etc. Even if the object is made invisible in the layers panel. Thus, That extra space you are seeing probably indicates there is some sort of object hidden in that white space. Try pressing Command+Y [Cntrl+Y] to view outlines. This will show you all the objects in your file. Additionally, the Object > Path > Clean Up tool works wonders for those who don't pay attention to the population and organization of their Layers Panel. When exporting objects in clipping masks, it will factor in the objects hidden behind the clipping mask. Therefore, I've found it BEST practice to use my artboards as my frame for the export, so I know EXACTLY what will be exported. So for your instance, you can select the artwork, navigate to Object > Artboards > Fit to Selected Art Hope this was helpful. Feel free to reach out with additional questions.
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