
I do normally split my rats' dry mix into two meals, one in the morning, one in the evening. I found that when I was feeding them all at once and then letting them graze on the leftovers during the day, some rats would graze earlier and more often than others, and I ended up with one thin rat and two increasingly fat rats.

15g per rat per day is the average for how much people advise you to feed your rats; if they're getting a full balanced cooked meal on top of that, they may well be getting too much. (Rats do vary a lot in how much they need to eat though; I've had does who needed 8g and bucks who needed 20g, and I've heard of rats who need even more because they live in outbuildings where it's much cooler.)
If they've come from a decent breeder, you could try contacting them, perhaps with some photos attached; the breeder should know how their lines normally develop and what kind of weights and body shapes are normal.