Board Thread:General Discussion/@comment-4957813-20130804122629/@comment-5398287-20140608183809

Sternritter M wrote: As far as I could understand, Rukia and Renji first felt the heavy reiatsu when they arrived at Hiyôsube's palace. It seems he possess much greater reiryoku than the other RG members, or something. And being the apparent leader of the RG, it would make sense.

If not that, there must be a much higher reiryoku level at his palace. Or his palace is just located closer to the Soul King's palace for some reason. Well, Cnet has this to say:

Rukia: It is true... // The concentration of spirit particles in the air here is quite irregular. // It no longer feels so oppressive that it physically weighs down my body... / But every motion I make here still feels like moving through water. // I cannot imagine what kind of effects all this spiritual particle concentration must have on our bodies...

The air isn't thick with Reiryoku/Reiatsu, but with Reishi. It seems to serve a similar function to the gravity chambers in Dragon Ball: weighing down the bodies of those who enter it so they grow stronger simply by striving to move about normally again.