From debian ISO faq:
Why is my downloaded DVD image smaller than 1 GB when it should be larger than 4 GB?
Most likely, the tool you use for downloading the image does not have large file support, i.e. it has problems downloading files larger than 4 GBytes. The usual symptom for this problem is that when you download the file, the file size reported by your tool (and the amount of data that it downloads) is too small by exactly 4 GB. For example, if the DVD image is 4.4 GB large, your tool will report a size of 0.4 GB.
Some versions of wget also suffer from this problem - either upgrade to a version of wget which does not have this restriction or use the curl command line download tool: “curl -C - URL”
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