Hi,
I would have guessed at a server restriction actually. We have users selling download files of several GB with HikaShop (games, movies), so unless your web server has some kind of limitation, I don't see why it would be a problem on your end.
Or maybe you use a really old version of HikaShop which didn't contain the optimizations that we have added in the past regarding the download of big files.
The download is handled in the file administrator/components/com_hikashop/classes/file.php in the function sendFile.