Fiberglass batt insulation is very squishy. Place R-19 insulation--a rather thick product--in a wall framed with 2" by 4" studs, and it will fit if you compress it with the drywall.
That doesn't mean you can use it, though. By doing this, you lessen the number and size of air pockets created within the insulation.
Use thinner insulation and you will get the same or even better thermal insulating properties.
You would want to use an R-15 maximum for 2x4 walls, and R-21 for 2x6 walls.