How the size of ribosomes are calculated?
Because Svedberg is not a measure of weight, but it is a measure of a relative sedimentation after centrifugation. 
"It is defined as the ratio of a particle's sedimentation velocity to the acceleration that is applied to it (causing the sedimentation)." 
The high number of S are correlated to the shape of the particles as well. 

https://en.wikipedia.org/wiki/Sedimentation_coefficient