For my physics coursework, I've been measuring magnetic flux density of different solenoids.

The solenoids are coils of wire wrapped around a hollow plastic cuboid (e.g. 5cm x 5cm x 30cm). I then stick a probe down the hollow centre which measures the magnetic flux density.

Im unsure about my results because as I increase the volume inside the cuboid (e.g. 7cm x 7cm x 30cm), whilst keeping everything else constant, the magnetic flux density value DECREASES. I thought the value would increase

Is this supposed to happen?