Don't be obtuse. What color was her hair? How tall was she? What was she wearing? None of that detail is present. This isn't scene-setting. It's a subtextual comment, and it's a clumsy and ignorant one. That's what bothered me about it: it's just bad writing. We get it, we get it: the author thinks lower-middle class families raise fat kids on junk food. Aha! There's one now!
He also mentioned that her father was "tall and muscular". What's the secret inner meaning of that?
A few small details are enough to generate immersion. The fact that you believe they were lower middle class in spite of the fact that he didn't mention it suggests you created a mental picture. I did too - in my mind the man was black, white t-shirt, a bit of grease on it. That's how the mind works, and writers exploit it.
A blue collar worker is lower class by definition. As phrased above, the "tall and muscular" bit is part of the writer's observation/stereotype that the man is a blue collar worker. "He was big and strong and (therefore) looked like someone who works in manual labour".
