Sounds more like a bug than anything that should be happening normally i would take some screenshots so as easier to demonstrate the problem and submit bug report you could also post in the Bug section of the forums.
When you change to the crab armor the backpack changes size, the problem is it doesn't change back when you return to the standard soldier armor. One could fix this by making it change when you change armor type, or make it an option for the backpack customization.
Bulky backpack
Slim backpack
I hope you can tell from these images
Well its been a while since i loaded my crab up and since my installation got fobared by windows i cant just go take a look but from memory there are 2 back packs one for archnoa armour and one for normal costumes.
The thing with the archons armour one is that it was made to work with the crab spider body type at a certain size and if you alter anything from the norm it go's weird
If you've not already try going to the tailors and making a small change to your chest or body scale slider it may fix it as it fixed mine when the pack was stuck inside my chest.
can't say i see much difference, maybe a more lighter screen would do?
I posted this awhile ago but no one gave me any response so ill try again, please post any thoughts you might have on the matter. . .
When you first get the crab backpack at 24 it is molded to the standard size of a wolfspiders armor and goes smoothly along the back of your character. However if you were to change to the crab spider armor (even if only temporary)the backpack changes size/color/angle and is stuck like that permanently for that costume slot, shouldnt that be something you should be able to modify, if you want to change back to the standard arachnos soldier armor?