bitcoin itself has nothing to do with an exchange.
inside an exchange you are not looking at bitcoins. you are looking at a database balance of credits that (ethically) should represent bitcoin.
meaning
the exchange is ALLOWING you to have some balance of their database. which if the exchange is ethical they will swap that balance for real bitcoins when you withdraw.
the exchange can do anything they want to their database. they can charge fee's just for having a balance, they can charge fee's for making orders they can charge fees for deposits and withdrawals. they can so anything they like to their database. including making it zero for any lame reason they want.
this is why people hate middlemen, banks, exchanges. with all the "we been hacked" lies, with all the negative interest rates, with all the bankruptcy issues you should not trust handing your funds to a middleman in exchange for their "credits"/"balance".
as for bitcoin itself. knowing eventually there will be only 21mill coins. you cannot make the quantity go down. you can however put them into an address no one can spend from, to make them unspendable.. resulting in less than 21million coins being spendable/in circulation.
now that answer the topic.
it seems like an exchange is messing you around, care to name the exchange, describe the events better?
have you contacted the exchange?
have you checked the trade history/transaction history within the exchange to see where/why the balance missing had gone?
This information about the exchange is very much informative to me, I am so thankful for this information. Anyway, bitcoin quantity for me will never go down as long as the user of it is increasing as time goes by.