For me it doesnt really matter if the story is well-known or not. If i feel like 'wow, this is a good story' i tend to want to comment and just say whatever. If i dont really like the story as much, then i dont really comment. I'm the typical silent reader, a little afriad for people to misunderstand and stuff, and only just now has started to come out of my shell to 'talk' to others on This site. But i feel like if i read something good, wether it is a popular fic or one of those thats not, if its new or old, the author deserves to know what i feel.
If youre wondering about why i'm suddenly commenting this much on your blog its because when i find good stories i tend to want to read more about the author and 'get to know him/her', so yeah.

Maybe people comment less because they think that the author might've gotten their comment already? That happens to me sometimes, like, sometimes I read a featured story that has many subscribers but I don't comment because I feel like everything there is to be said has already been said and I don't want to sound repetitive… but there are also times when I read half-way and don't have a mind to comment and then forget.

I dunno - I don't have any of mine featured anywhere (that I know of), am not very well known (again, that I know of) and don't have many subs/comments/upvotes/anything. I'm not necessarily bothered by this (cuz I'm weird like that) but it kind of bucks your trend a bit. I just put them here for somewhere to put them where I can actually see whether anyone is reading them or not. But then I kinda forgot to look cuz I haven't had the time to update anything since I got pregnant and had S. *fails*

I think for some (i mean a lot) may or may have not stumbled upon popular fics, and they subscribe it, but they don't actually read it. They just subscribe and include it on queue list. I think that's the most logical reason to explain the imbalanced ratio.
Also, I think there are too many inactive subbers or even subbers that don't have enough karma to upvote and that would explain why the upvotes are way fewer than the subbers.
Furthermore, I think there are quite many subbers that are too picky and only upvote if the story is in accordance with their personal preference.

However, I can't find any explanation on why less popular fic would get so many upvotes though. But the one that commented below me gave a very logical explanation. People tend to make interactions with less popular fic to make the interaction more personal and intimate. And I sometimes also give mpre love to certain stories that I've subscribed before it become popular. So yeah i keep babbling -_-