Ratchet can't store his weapons in Clank, because he doesn't stick around Clank all of the time while you control him.
Ratchet can't be Batman because I said so.
Ratchet can't shrink his guns, because that sort of technology would be going to waste on such a task, seeing as how shrinking stuff would be better applied to a weapon he could use to defeat enemies easier.
Ratchet can't use nanomachines to shink his guns because where does the matter all go to?
Ratchet can't use a controlled Black Hole to compress his weapons, because there is no way to reverse this process, and if there were a way to do this, I'm sure the universe would be a very different place (I'm not a scientist, I don't know these things for sure)
Ratchet can't store his weapons at all, that technology doesn't exist in his world, or ours. He's just special…
Which means that Ratchet's super power is: Storing weapons! It's just a shame no one ever notices this, including himself during cinematics.
While we are at it, we could go on about how the Shrink-Ray in Size Matters is completely flawed, and Ratchet could have ended that game much faster than he did… but what fun would that be? Ratchet & Clank use cartoon logic: Noting makes sense, accept everything and enjoy the ride while it lasts. Just like with the fire in Spongebob.