I'd reckon that the Chinese solved the weight problem by their usage of those "less advanced optics".
The Ballistic computer/Thermal telescopic sight of the XM25 for example, weighs 1.15 kg alone.
If China did in fact replace that Sensor suite with a less advanced and thus lighter one, that might put the weight at that 5 kg number. 5 kg isn't that far off from "normal" actually, as the XM25 weighs only 5.45 kg unloaded and without the Sensor suite.
Just a quick add-edit, for those who don't think that OICWs are important for the near future, here's a quick graph of Realistic combat ranges the U.S. Military faces in Afghanistan:
So it's quite obvious that there is no small-caliber Infantry assault rifle in mass service today that can deal with those ranges, other than say, a OICW, with the XM25 as an example, it provides over 700 m of lethal carnage.