I am not sure we are at the point where current retro format is not working. Indeed, 17 people are usually too many for the meeting, but retro is kind of special "get together" meeting so I am concerned whether it won't create kind of team fragmentation so we have no idea what people in other sub-teams think, their perspectives, etc. I think our sub-teams are small enough that we can solve intra-team things just by talking to each other, but retro is for bigger things to be aired and everybody's chance to talk about them.
So I am ambivalent on this.