This strikes me as creative framing - trying to spin a clear negative as a positive and then shoehorning the word "growing" in there.
Obviously, the clear issue is that demand is falling for that segment. They have instead chosen to spin this as "a different segment is popular and causing substitution" which seems like a dubious claim given the obvious differences. It's not like a switch from truck to SUV which might make some sense. But they didn't stop there either. They are calling it "growing market change" which almost makes it sound like this is a good thing.
The reality is that the sectors are almost 100% independent and this is unequivocally bad news. There's no reason whatsoever that both segments couldn't be growing at the same time. It'd be like Tesla blaming low EV sales on growing solar panel demand - almost totally nonsensical.
Someone buying solar panels cannot drive them to work. They would still need a car, but someone buying a Crossover/SUV/truck would not need a compact car. Given a choice, every automaker except Tesla would choose 'low gas prices-high truck/SUV sales-Low EV/Small car sales' to 'high gas prices-low truck/suv sales-high ev\small car sales'. Truck/suv sales paired with low commodity prices have helped GM double their net income every quarter so far, from an already record year in 2015. The market change they mention is a nice problem for them to have. Ideally they would like to sell more small cars too but in a limited, saturated market, a $23K crossover sale is preferred to a $16K compact car sale, both based on the same platform. The sectors are not 100% independent. A sub-compact Sonic buyer is not going to buy a Suburban but is very likely to buy a sub-compact Trax.
They have also voluntarily sent 100,000 fewer cars, mostly the Cruze and Sonic to rental agencies in 2016. Their other small car sales are in the dump even in retail sales.