You're imagining something like a thief who just intends to steal $X, robs a bank, counts out $X and leaves the rest of the money untouched. In reality, most thieves are opportunists: they will take as much money as opportunity allows without getting caught.
Obviously you couldn't physically fit $1B cash in a wallet, but assuming this hypothetical wallet did have $1B, does that make the thief more heinous or just luckier?
(If you must insist on a literal and physically accurate wallet in the hypothetical, just imagine it held $1B in Bitcoin.)