I stumbled upon a web site that counts conference publications and citations. I asked for their count in Algorithms and Theory conferences, looking at the last 10 years. Here is the result.
This data gives some idea of what it means to have a paper accepted at one of those conferences.
For example I have always thought of CONCUR, COCOON, COCOA as conferences of roughly similar prestige: I have never published there and have never attended one, but I know people who do go there, and the names have so many letters in common that they're almost fused in my mind. Now I see that a paper at CONCUR is much more meaningful than at the other conferences with similar names.
The biggest surprise for me is the contrast between RANDOM and APPROX. I've always thought of them as twin conferences, but actually, RANDOM has much more impact - perhaps because it is better targeted. APPROX is broader and its scope is harder to distinguish from, say, mainstream SODA papers, so it only gets the leftovers after SODA has been served, whereas RANDOM, perhaps, gets some specialized papers that are sometimes a little bit marginal for SODA but that are still very nice papers that will be well cited by people in the area.
If you hit a niche and have the leading conference in a particular subtopic, then your conference can have high impact even if the number of papers and of attendees is relatively small.