The intensity for any pixel in any detector is determined by the energy hitting that pixel and the size of the pixel. If you change the size of the pixel, you may gain or lose energy faster than you gain or lose area. So if you change the pixel size to be slightly larger and suddenly one of your pixels happens to intersect a lot more rays, you could see an increase in the intensity. But if you pick up no new rays and the area is larger, you would see an increase. Generally, this is why you want to have a lot of rays, enough to get many rays per pixel in the region of highest intensity. 

Hi Kevin,

Thanks for your response.

I understood how the Zemax detector works but for a particular luminaire peak intensity and beam angle should be same. How to achieve the correct value for that luminaire using Zemax? because in LAB using goniophotometers results would always be same for a particular luminaire.

This is a major issue for small beam angles like 15 deg.

Currently I am using 10M rays.

Please suggest a possible solution.