I've heard people say, "Oh, it's a small audience at that workshop. People will be very talkative."
Not in my experience. I've found that large audiences encourage more discussion because audience members are less conspicuous; they can hide in the forest. Say something in a small group and you may as well be on stage.
Large groups are less inhibited. They are ready to rock and roll. They're away from the office drinking coffee, checking out one another, and planning lunch.
Small groups that have worked together can be similarly uninhibited but small groups of strangers are about as wild as a church social.
Consider those factors when preparing a presentation. It's not that you cannot get a small group to open up. It's just that it will take more work.