Good read here - https://www.surefire.com/how_to_choose_weaponlights
There are several factors that effect the width of the hot spot in the center of the beam. One of them is the size of the LED. All other factors equal, place a larger LED in the same reflector and the size of the hot spot will increase. A larger reflector would be required to maintain the same beam configuration as a smaller LED.
Larger LEDs are used for higher current capacity and lumens. Could very well be that SF is using a different (larger) LED in the new X300 for the increased lumen output, which would fully explain the lower candela but wider hot spot beam pattern. But since SF does not specify which LED is used in their product, that is speculation on my part and the only way to know for sure is to put the new version next to the old one and observe the size of the emitter.
For light weaponization purposes, there is no dispute that the more narrow, higher candela/lux beam has the greatest blinding potential, so in that regard the 600-lumen version still has the edge. However for general illumination purposes, distributing the lumens in a more even pattern while maintaining a discernable (if less intense) hot spot is generally the best compromise.