Ok, I like the above two answers a lot ... since they say what I want to hear. However, here is what SR SunTour responded to my official support request (after less than two hours of the request!):
Dear Mr. Vitkov,
the leverage is much higher if you go for a bigger rotor than 185mm.
You risk that the fork breaks if you go for a 203mm rotor!! So we
strongly recommend not to do this if you don´t want to get seriously
For most riders a 185mm rotor is more than enough.
Best regards Benjamin Rees
I still do not know what to believe. For example, I do not know of 185mm rotors, the closest I have seen is 180mm.
Another controversial point is that their more expensive forks - EPICON and AXON also are limited to 185mm, I managed to locate only 1 fork limited to 210mm - a dirt jump intended one.
One possibility that I can think of is that the stanchion tubes could brake off the lower tubes at very high deceleration. However, deceleration is as much function of the rotor size as of the brake type e.g. high-quality hydraulic or the cheapest mechanical brake.