Need help: HDR Reflections changes by changing output size

[floschaerfer]

Hello,

im an Industrial designer based in Germany, Cologne.

Untill now i thought i knew Vray for Rhino basically but now i cant solve my Problem alone so it would be very nice if someone could help me.

I wanted to render an object (not commercial) with some highlights on the surface so i add the dome light with a hdr texture. Further the object has a semi matte material with a fine bump to get a grained plastic appearance.

 

The Problem is when i change only the output size also the reflections on the object changes.

When i render in 640x 480 the reflections are like i want them to, but if i render in 3543x 3985 the reflection is soft an without highlights. (Shown in the picture)

For Primary Bounces i choose Brut force for Secondary Light Cache. I tried to change the light cache subdivs and scale, the hdr texture resolution and intensity but without success.

I cant see the connection between hdr reflections and output size so please help me