Much more to it than the size of the lens, the heart of a digital camera or a thermal imager is a big factor in the quality of the image....
As with a scope, camera, or whatever uses optics, the quality of the lens makes a big difference. How the thermal imager processes the image is another factor, one which, like computer processors, is making leaps and bounds in the industry.
A digital camera uses a CCD array to process the image and of course different CCD's have much different resolution capabilities than others.
In the basic sense, thermal imagers use an infrared detector phased array of elements to create a temperature pattern/thermogram. The temperature pattern is then translated to a signal processor and basically sent to the display so that you can view. The detector array and signal processor technology is advancing, hence why we see better thermal scopes as technology advances.