This magic item will store 500 more pennies than it appears to be able to! Any advice on how prevent it from breaking any campaigns?
How would it break any campaign?
Keep in mind that it isn't hard at all for an experienced magus to create a large amount of money out of nothing.
The fact that is bigger on the inside has normally made people nervous. If instead you want an Imaginem effect to make it look smaller than it is, coupled with a Mu(Pe)Te* effect to make the conetents lighter that might work, and be fine.
*I never know if I need a Pe requisite for destroying weight with Mu...
I'd go with Muto Terram where it reduces the size of any metal items placed inside. Breaks no limits.