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.