Get the biggest features of images

[KirbX]

Hi,
This is not an issue, but it's a question.
I heard somewhere that using SIFT match, we can do a first pass of pair matching using only the biggest features and the results are proportionnal to the end result. It can be used to optimize a little, because we can pair match with the 100 biggest elements for example and if the pair matching is bigger than X%, we do the feature amtching with all the feature of the 2 images.
(I'm not sure if this is understandable, sorry)
My question is, using Sift-GPU, how can I easily have the biggest features in an image?

Best regards,
Raphaël S.