OP didn't say he/she spent a significant amount from his/her 1 BTC investment. What he/she said is that he/she invested 1 BTC but the last time he/she got access to the account, only 0.038 BTC is left. And so he/she asked them about it. He/she wouldn't have made a query about this if he/she was the one who spent it. It was gone because it was stolen by these scammers.
Large is relative. But to be safe we can just generalize that any service that asks for Bitcoin, large or small, is a scam.