At the risk of being pedantic, is a "piggy bank" really the right metaphor for this? A piggy bank has actual money within it, regardless of what I think of the piggy bank, it still has money within it. While a stock's total market cap technically only represents the last transaction price multiplied by the number of outstanding shares (kinda). Anyway, just making the point that it isn't like a "bank" in that there is not money in a container of any sort (obviously, banks lend out their own deposits so even my correction is also wrong).

I'm don't have anything better to replace it with, but I am just questioning the "piggy bank" as a metaphor here.