Your weight or the size of you head has no bearing on the forces. G forces are about acceleration and deceleration. Your deceleration will be the same regardless of your weight.

1.5 G's against the seat belt means a body that weights 150 pounds will exert 225 (1.5 times 150 lbs) against the seat belt. The 1.5 G limit is due to the limit of braking traction the tires have. Your 155 pound body would exert 232.5 pounds against the seat belt.

It is not the weight that damages. It is the rate of deceleration.

There is a saying. Jumping out of an airplane is not dangerous. It is the risk of a sudden stop that is. The fall doesn't kill you. The sudden stop does. A stunt man can jump from extreme heights if he lands on an airbag that slows his deceleration at the bottom..

If you do not understand the basic of the laws of physics, then you need to just try to accept that I do and you did not sustain a concussive force or even a sub-concussive force.

PLUS, it takes HUNDREDS of sub-concussive forces over a few weeks or less to cause a cumulative injury.