Suppose I have 1 BTC.
I send my 1 BTC to an address XXX that has a balance of 50 BTC.
Next, 5 BTC are sent from XXX to YYY.
Where is my 1 BTC now? Does XXX or YYY have it?
The answer actually depends on the choice of inputs for the transaction from XXX to YYY. However lets assume that XXX's controller sweeps all inputs into a single TX because I think that's the gist of your question.
XXX at first with 51 coins has 1 coin from you and 1/51 of it's balance is from your coin.
YYY then gets 5 BTC from XXX, thus 5/51 of your coin is there.
XXX still has 46/51 of your coin.