just something i'm working on..

[Kelvin]

I think Prime (or something? not sure) confirms that her arm is actually inside the cannon. I'm pretty sure the X-Ray visor in particular shows her hand if you try to look at her shooting arm.
Plus it doesn't make much sense that she can become Zero Suit and still have a regular arm if it's cut off in the Power Suit.

The thing that bugs me though is not just in old sprites, but new ones and even some 3D media, the cannon arm is shorter than the non-cannon arm by a full hand (literally). Even if you made the cannon longer, it doesn't explain a lot of other things about how it works.

I guess if Chozodian technology can fit you into an armored sphere one third your original size, they can compress an arm or store it in a subspace pocket or something.