Siblings

Lately something slightly odd keeps happening to me.  People keep assuming that I am an only child.  I have multiple siblings, some of which are full siblings and some of which are step-siblings.  This led me to wonder, do "only children" give off some sort of vibe?  If so, how do siblings change this vibe?  Is there some stigma that goes along with being an only child?  Should I be worried that people think I'm an only child or is it a compliment?