Well done for setting boundaries like that!

I keep reminding myself and my friends that, unless your work literally means life or death for your customer/client, our work isn't so important that you need to work hours and hours of overtime to finish it instead of completing it the next day. And if the amount of work you receive each day is larger than the amount you can complete, it is a pretty safe sign that you need more personnel (provided your time management already is good).