Without knowing the KB Article - I would assume that you are right and this is an SQL Service pack based purely on the file size.
If you can't install through windows update, can you download the SP and offline install it?
https://technet.microsoft.com/en-us/sqlserver/ff803383.aspx
The above link gives you the option to download the SQL service packs and then install them manually whenever you want to. This will remove the wait time when you are downloading through Windows Update, will allow you to install at a time to suit you and (if you save a copy somewhere) save you the bandwidth of downloading it for every individual SQL box you have.
Remember to backup your databses first!
EDIT: Based on your comment with the KB Article (KB3072779) - the article I linked to above will direct you to the following download page: https://support.microsoft.com/en-gb/kb/3072779
If you go about half way down the page and select your SQL version (Express, standard etc) - it will send you to a direct download page where you can get your service pack