Once you give out its address for someone to send you what they owe you they will know, which is not the case with your physical wallet or your bank account.
They'll know how much is in that address, but a wallet consists of many addresses. How do you find all of the other addresses?
If you watch for long enough, you can wait for a transaction that spends from the address you know, and you might find some other addresses that were spent with it.
It's true that a non-zero amount of information about your Bitcoin holdings is leaked. But it's not true that that amount is 100%.
How much money is in my Bitcoin wallet?