I've also had BCs who were really skinny when young. I have a photo of our Jess when she was about 2 years old and she looks so thin, it's untrue. She doesn't look bony in the photo though...just really really lean. She filled out when she got older, her ribcage sprang and such....became more curvy...sort of like a woman, lol.
Her dad was just the same. Always really lean. Some people thought he was underweight. It was just the way he was. He also filled out when he got a bit older.