I ran Boston 2013 (finishing in 2:57:xx). Walking around the athletes village on the morning of the race, I couldn't believe how crowded it was. I was afraid of stepping on people laying on the ground, catching some zzz's before the race. In my mind, there is no way that another couple thousand people could be squeezed onto that field. But what about the 100th anniversary, when Boston hosted a ton more runners? That took 2 years of planning and a second, overflow athletes village that has since been built on. In my mind, the village is the number one limiting factor.
I'm in the white hat and white singlet directly below the clock. Courtesy of Boston Marathon FB page
Second, the start line is a small, two lane road. Chicago hosts 45,000+ runners but the roadway is huge! NYC hosts even more but the bridge is 4 lanes and a double decker. Drive down a backcountry road in the middle of BFV, and you have the start line at Boston (I am in no way comparing Hopkinton to BFV, USA).
Whatever happens, it is a guarantee that Boston 2014 is going to be epic. I am all for adding a few thousand people (on top of the 5,000 who are guaranteed entry) but to go for 45-50,000 is simply impossible for the BAA to handle. I'll support whatever the BAA decides on, I will support it. Good luck to everyone training and attempting to qualify for Boston 2014. I wish you all the best of luck. I'll see you in Hopkinton!