He might mean it in relation to the other people he saw. The others allegedly there were a few years older than Caroline so presumably a bit bigger and she could be called a "wee lassie" compared to them.I'm a bit confused about the idea of a 4 year old describing a 14 year old as a "wee lassie". I'd have thought that, at the time, she'd have been a "big girl" to him. But I suppose he might have been parroting the words of the older ones. Or maybe he just wanted to sound big himself... It must have been very traumatic for him. He said he "closed his eyes" when she went into the water.