Unzip on the QNAP is unfortunately the "unzip" included in busybox, and busybox on the QNAP is ancient and really outdated.
This will cause that large zip files might cause an error:
Unsupported compression method 0
Replacing busybox, on the QNAP, appears tricky, and can cause possible problems. (see this thread in the QNAP forum)
The only "good" option I have found so far is simply installing OptWare, which is outdated as well, but allows you to install "unzip 0.6.-2".
After installing OptWare, go to your QNAP Web Gui, open "App Center", and locate "Optware ipkg" and click the "open" button.
In the upcoming page, at the filter field, type "unzip" and press ENTER (or click "submit").
A list will appear, in which you see a green "install" button just before the "unzip" line, which will install the newer unzip.
Now to use this new unzip on the command line (SSH), say to unzip a very large zip file called "archive.zip" do this: