Stretch your balls out with ball stretchers.
Over a period of months or years, this will make you grow additional scrotal skin...
Once you have a lot of scrotal skin, you can inflate the scrotum with saline solution... this temporarily makes your scrotum huge and heavy.
Its not really making your actual testicles any larger... but there are guys who make their scrotum enormous with this technique.
Within days the saline solution gets absorbed by your body and things go back to normal.
But I have seen pictures and video of guys who wanted permanently large and heavy ballsacks use silicone rather than saline.
This is a permanent mod... no safe way to get the silicone out... and no real data on health or safety of having it in there...
And too much can really leak into the tissues around your dick and make ordinary sex impossible...
but you can see vivid pictures of these kinds of mods on the BME site.