Use DISM / Export-Image para criar um novo WIM e isso reduz o tamanho:
/Export-Image: Exports a copy of the specified image to another file. The source and destination files must use the same compression type. You can also optimize an image by exporting to a new image file. When you modify an image, DISM stores additional resource files that increase the overall size of the image. Exporting the image will remove unnecessary resource files.
Dism /Export-Image /SourceImageFile:install.wim /SourceIndex:1 /DestinationImageFile:install2.wim /Compress:maximum