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It's much more work to calculate RC and so on. Custom Json which is what is most of SM/SE is more expensive than votes and transfers not quite as much as post/comments. So it will still be a large %.

The activity is more like 28-34% when you factor in transfers to SM/SE services.

I'm not saying it is bad that it uses a large chunk, just was curious. I do think it means you should have your own nodes though.