In reply to styman: I just copid this from a thread on rockclimbing.com ;
I just finished a series of drops on half ropes with 80kg at the request of Will Gadd.
Rope A. 80kg-7.35kN, 55kg-5.39kN, published with 55kg-4.85kN
Rope B. 80kg-8.15kN, 55kg-6.23kN, published with 55kg-6.3kN
Rope C. 80kg-8.23kN, 55kg-6.25kN, published with 55kg-6.5kN
Rope D. 80kg-9.22kN, 55kg-5.88kN, published with 55kg-6.1kN
These drops were conducted without the regulation conditioning but complied with all other requirements and procedures. Relative humidity was 42%, temperature was 20ºC for 48 hours. I also listed the published numbers from each manufacturer; I used ropes from 3 different ones.
http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?post=1394069;sb=post_l...
And again from Jim
It's not as simple as it sounds. A skinny rope does not generally stretch more. The construction of a skinny rope is different than a fat one. Because a fat rope has so much more mass than a skinny one it has to be designed differently so it won't give too high an impact. Skinny ropes have the opposite challenge, more or less. Half ropes are drop tested with 55kg but are tested for static elongation with 80kg so they still have to offer a high resistance per mass.
Manufacturers do all kinds of things to manipulate the properties of the rope so it's not just a matter of diameter or mass, although that does come into play.