dism /Export-Image /SourceImageFile:C:\mount\install.wim /SourceIndex:1 /DestinationImageFile:C:\compressed\install.esd /Compress:recovery

If you genuinely need a smaller download:

No legitimate tool can compress a 4–6 GB Windows 10 ISO into, say, 200 MB. The official ISO is already compressed using LZX (a compression algorithm). Any "highly compressed" file under ~2.5 GB is either: