We take the case where a vehicle is going around the banked track (bank angle = theta) at just that speed at which it tends neither to fly off the track outwards nor fall inwards.
Since it is turning in a circle, its velocity vector is changing direction, which requires an acceleration towards the center point of that circle. The acceleration is furnished by the normal force developed at the tire contact points.
Since the track is angled, so is the normal force, which points upwards (due to the reaction force furnished by gravity) and inwards (due to the radial component of acceleration, which is required to turn the vehicle in a circular path).
The resultant acceleration vector is greater than that furnished by gravity alone because it is the vector sum of gravity and the radially-inward directed acceleration which is turning the vehicle's velocity vector.