As I understood it, Remilia was born a vampire to the noble House of Scarlet, and her father was also a natural born vampire, in a noble line stretching back a long ways. I assumed, therefore, that since she grew past being a baby that she's still growing, just very, very slowly, and will one day appear as an adult, like her father did. Also, unless she's unfortunately sterile, she would be capable of furthering the line of the Scarlets. (Or is it only male vampires that are fertile? If so, Dracula's tendency to collect vampire brides seems even nastier.)

Sorry again if I missed something in the text that explains all this.