That's a great point, verstrickt. The most enjoyable conferences for me are those small-medium ones. I prefer conferences with on-site housing and meals where I have to sit next to someone in my field each day. The bigger the conference, the more likely I am to cling to my little group of former labmates for emotional security, and then I get less out of it.
Small conferences are definitely the best. Even better is if the small conference has even smaller workshops or (in some fields) field trips before or after - a day or two with a small group pushed to interact creates connections without "networking."
I find that most of my network expansion comes from my existing network. I almost never approach people I haven't met, but I have been introduced to many many many people just by virtue of being a mutual acquaintance. One such introduction led to a collaboration, and that collaboration has led to a fantastic postdoc. All because I happened to be standing at a conference talking to an acquaintance that my now-supervisor wanted to greet. That acquaintance was a former PhD student in my grad school department who graduated three years before I arrived, but who I knew through the transitive socializing of the department's students at conferences.
Leading to the next point - I don't know about other fields, but in mine student-adviser relationships are often quite informal, and introductions can go both ways. If you're a student/postdoc, don't just think about networking with more senior people, but try to build a really strong network with your peers (especially from other schools/departments) as well. Mostly because these will be your colleagues for the rest of your career, but an advantage is that they will have different networks of more senior people than you do. I recently made a connection with a bigwig because I'd connected with (and had some great discussions with) one of his grad students. Another senior network node that may lead to collaboration came from a fellow postdoc mentioning to his former masters supervisor that we had a mutual interest.
I feel like it eventually kind of snowballs - the bigger your network, the faster it grows and the easier it gets to manage conferences (in terms of social anxiety, not in terms of business and demands on your time!). I've been going to a very large conference for the past 6 years. It's in Europe, so mostly draws a different crowd than I knew from my grad school days. The first year, I knew only a few people there (like 5 slight acquaintances out of 7000 people). It was kind of awkward, I mostly ate alone, didn't really meet many new people, and it wasn't so great. The next year I knew a few more, then a few more, and by the 6th year, it was a super productive meeting, I feel really plugged into the community, and my network grows like crazy every year. Plus, the conference is now really fun - I've built social connections as well as scientific ones, and now have some great "conference buddies."
Benefits so far have been a job, several collaborations, invited talks, many really useful science discussions, tons of useful advice (technical and career), probably a bunch of citations, help with field logistics, offers of lab analyses, and a name that's increasingly recognized. I think it's worth the effort.