The weight argument is 100% irrelevant in high level professional rugby today.
If you want to talk about physicality, then let's talk about explosive power, not weight. Paisaimi is listed at 90kg, which makes him a lightweight too. Yet he's very physical.
And even still, not everything in a backline is about the explosive power of individual players. Technique (for passing, kicking, tackling), mobility and cohesion are huge factors as well. Stewart and Flook aren't athletic beasts, but they are quality rugby players and at the end of the day, on a rugby field, that's what matters.
As far as defence is concerned, again the weight argument isn't particularly important for a center pairing. What matters is tackle technique, positioning, and communication.
I think this Reds lineup is the best they could select and I'm no worried about Stewart / Flook. Because it comes with JOC (James O'Connor) / Stewart at first and second 5/8 (they play great together), and it comes with a game winner (Petaia) on the wing. Flook is a good link man and a potent attacking threat on his own.