Teen and especially preteen girls tend to be really skinny until they put on their adult fat. I remember fitting into a size 0 at 12 and 14, same with my sister, we were both healthy.
Then I looked at my yearbook about last year and I notice something, many girls were very very skinny, just like I was at that age. Many seemed to have the same kind of body. They didn't have the more adult body I (and many others I'm sure) grew into. In an unrelated conversation I had with a friend, she was mentioning about how her body has changed since she was in high school.
You can't really compare a child with an adult.
As a hook for the article I thought it was clumsy and unnecessary.
I think it's reasonable to say that this introduction suggests a middle class snobbishness towards the lower class. The worry is that by not teaching the kids correctly, the kids will end up in the lower class.
On the other side, it's also teaching people that they shouldn't want to be in the upper class, because then they would lose this important sense of money.
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".