Why might a full-sized unused structure reduce an organism's fitness? If the vestigial structure was the same size as it was many years ago, energy needs of an organism would have to be greater which, depending on the environment, might be hard to accomodate. It does not matter how large the vestigial structure is.

full-sized unused structure, also known as a vestigial structure, can reduce an organism's fitness because it requires energy and resources to maintain without providing any benefit. This can lead to a waste of energy and resources that could be used for other essential functions. Additionally, the presence of a vestigial structure can be a sign of evolutionary changes that have occurred over time, indicating that the structure was once useful but is no longer needed. This can be a sign of a lack of adaptation to the current environment, which can negatively impact the organism's fitness.