(xpdev)

Quote - ...what the hell I'm wrong ?

     Not wrong, you've simply discovered the relationship among emitter size, number of ray trace bounces, irradiance caching, indirect light quality, and splotchiness. You also might notice the quirky Firefly IDL response in corners.

     A large emitter can use a quite mild ambient value, and the lighting tends to be even, without splotches. A small emitter needs very high ambient (Poser seems to clip values over 30 or 32) and tends to cause splotches because sampling rays often "miss" the small emitter. I try to use an emitter which is just large enough to avoid splotchiness, and then use several ray trace bounces and engage GC to help even out the light. 

     If you use an overly large emitter, you lose the appearance of the light coming from a particular source, one aspect of which is directionally-oriented shadows. Ideally, the emitter should fit the visible light source, so that the lighting matches the apparent sources. Also consider that most lights aren't searing blown-out white. Consider making the visible light a gentler ambient (0.5-1.0) and using a corresponding unseen emitter with sufficiently high ambient to cast enough light.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5