Can someone explain this joke? Don't all magazines have springs in them?

Going by the commentary which DeepL translates to:

A magazine holds fewer bullets than its size

I assume it's nothing more than spreading awareness about the existence of the spring.
Maybe people are forgetting it in artwork?

That, and I'm assuming a lot of people just thing the magazine is "all bullets", rather than "bullets, and things to make sure the bullets go into the gun".

artists knowing what a follower is=win