I noticed it about a week ago, was looking for a way to confirm it before posting my suspicions :) And by the way, who decides that it is a lightweight story?
Lightweightness isn't decided per story. Whole sites and filetypes are. At the moment it's just me, because the code is simply a list of each: there's no mechanism for adding new ones except editing the source.
For adding new sites to the list, how would you favor incorporating a small positive weight for sites which are not on a "positive weight" list but that have a history of generating votes (or comments/comment points/comment word count) above some threshold. The weight could be very small at first, but be additive until it meets a value which could include it in the positive list (or flag for a review queue). In the same way, sites which fall out of popularity can be slowly and moderately down-weighted.