​What does Tom mean when he mentions that there is not much difference in size of Lucy and willie ?

Tom is referring to the fact that Willie is very small for his age, particularly his weight.

A chubby six-year-old with brown curly hair, earth smudged over two enormous pink cheeks, was standing at her side holding on to her skirt.
"Don't be so daft, girl," she said. "Go on, say hello to him. I got things to do."
She clomped down the steps and stood shyly beside Willie, twisting the hem of her dress in her hand till her knickers came into view.
"There ent much difference in size between them two," said Tom, observing them together. "I dunno what they do with little 'uns in that ole city."