However you can structure transactions such that you spend all inputs in a specific address to avoid address reusing.
I think this is what they do by default.
I am fairly certain that they will only use the necessary inputs. Although they may use the smallest inputs and then move to larger inputs as necessary which may make it look like they are using all inputs depending on the inputs in your address.
Both using all inputs and using the smallest inputs first will most likely result in you having to pay less in tx fees over the long run.
If you give me a few hours I will take a look and test my theory.