Melantha is a 28 year old being of darkness living in the sonic world. She can alter her appearance as she sees fit, the only thing she can't change is the lack of ears. The size depicted here does help with subverting expectations. With Mephiles having the "speak no evil" motif in design, she has the "hear no evil" hence the lack of ears (despite obviously being able to hear). She has the element of darkness, part of which she uses to alter her appearance (the size she over all defaults to will tend to be pear shape after she's done subverting expectations). In some regard, she may have had a hand in the freeing of some of dark gaia's minions (auveata, majhi's family, teranis, yivzari), whether they realize it or not.