Download corruption is caused by either your LAN set-up or issues on your ISP's backend. Delete the corrupted download from the Downloads notification list. Run safe mode option 5. Power cycle all your network hardware (router and/or modem). Stop using WiFI and switch to the vastly superior Ethernet (wired) connection, if not already doing so. Stop using your ISP's DNS servers and switch to either OpenDNS or Google DNS. Try downloading the game again. If issue persists, then contact your ISP.
As for the download size thing: all PS4 game downloads from the PS store are in two parts. What you download from the PS store is the pre-load, which allows you to start playing the game while the rest downloads in the background. The actual (non-patched and no DLC included) size is shown for the first minute or so of the download, before reverting to the pre-load file size. To check on the download progress of the application (rest of the game) data then highlight the game's tile on the XMB and press the Options button, then select 'Information' on the menu that appears.
