In a real car there is no such thing as x% braking; it's a linear relationship between the force applied to the pedal and the force applied through the braking system. G25 etc pedals operate on the basis of there being a linear relationship between the distance you move the pedal and the force applied through the braking system which is not the same thing. The brake curve factor setting is to try and apply a nonlinearity to your pedal input value to match the first scenario. Mine is always set to 0 as I have a load cell brake pedal.
Indeed my brake pedal hardly moves at all beyond the initial phase of braking (pushing the callipers onto the disc), it's how hard I push it that makes the difference - very natural, and very subtle as you can control how much force you put through your foot and leg much better than how finely you can move your foot.
In the real car there is a lot of pedal force required, so you're able to put more force into the brakes in the sim because you just need to move the pedal against a low resistance spring. You just don't have the ability to finely adjust your braking because of the nonlinearity inherent in travel-based pedals - that's why you're locking up. To address it, tweak the brake force curve factor to try to correct that nonlinearity to give you a slightly larger window to play within at the limit. On a load cell brake you just feel it naturally and make a tiny muscle adjustment - that's how it should feel.
The real drivers may well brake as hard as they can but that doesn't neccessarily mean that they're at what we know as 'full brakes' in the sim.
Similarly some cars have a lot of torque in the steering column, but because of our equipment shortfalls, we have to make do with much lower forces through our steering wheels. Just as we don't need as much force to turn the wheel, we need less force to push the brakes - it's just an unfortunate reality of simracing that we have to put up with. Instead of altering the feel of the brakes to accommodate these shortfalls though, iRacing have kept it as accurate as possible and let us try to improve the situation at controller input level, otherwise it would be totally wrong when the hardware shortfall is rectified.
For example, in race 07 if you set the car up with "100% brake pressure" (which doesn't exist) then it's still difficult to lock the tyres up fully - and I have to brake very hard with my pedal to achieve a low resulting pressure - not linear at all. It's that sort of bad practice that iRacing try to avoid, it should be addressed at input level rather than in the physics engine.
So, tweak that setting and bearing all of the above in mind, be prepared to brake 'less' (or should I say move your foot less!). Or buy a set of CST pedals or equivalent, as in my opinion and experience, a load cell brake pedal is the number one thing that'll improve your simracing experience.