Ok, I ran the test 
a) paper size DINA5, Rastral Size 3 (7mm staff), export as .png 600 dpi
result: graphic is 3300 pixels wide
b) paper size DINA3(!), Rastral Size Custom (14mm staff), export as .png 600 dpi
result: graphic is 6600 pixels wide
c) a screenshot of this
result: graphic is 1100 pixels wide
a) and b) it looks to me as if resolution is directly dependent on the set Rastral Size