Dear @hakan.cetinkay,
The ratio between both gamma line intensities is around 1.8 as you can see here. In which proportion are these gammas detected depends on the efficiency of your detector, which is related to its geometry, and therefore one cannot simply state that “in reality this is about 8”. If you play with the size of the DETECTOR region in your input file you will see that the ratio of the counts for each line will change, as it should, since the proportion of gammas that deposit their full energy in that region will evolve differently for 511 keV and 1274 keV.
Hope this helps. Let us know in case something does not add up.
Kind regards,
Francisco