The relationship between the size of an object and its portrayal on the image is not linear because:

The correct choice emphasizes the concept of modulation transfer function (MTF) in imaging systems, which describes how well the system can reproduce (transfer) various levels of detail from the object to the image. MTF is crucial in understanding image resolution, which varies with object size.

Larger objects tend to be perceived more accurately in imaging systems partly due to the MTF's behavior. The system often captures and accurately depicts the spatial frequency data better for larger objects compared to smaller ones. This results in a non-linear relationship where the ability to visualize detail diminishes for smaller object sizes due to limitations in resolution capabilities of the imaging system.

In contrast, the other options suggest that larger object resolution is universally superior or imply a consistent resolution for all sizes, which does not reflect the true behavior of most imaging modalities. The intricacies of MTF make clear that an imaging system's ability to represent finer details is contingent upon various factors, including the size of the object being imaged. This understanding is pivotal in optimizing imaging techniques based on the size and nature of the objects under examination.