An interesting article on Co.LABS highlights the significance of scale with platforms like Twitter. When you send one package of data off to another person, that seems like a pretty simple thing with the internet. But Twitter is a vastly upscaled version of that - 150 million active users (the most active ones probably fire off a tweet at every moment's whim). That's a lot of packages to send through.
When things fail, you end up with the 'fail whale' page and although the meme is cute, the situation of not being able to use the service definitely isn't.
Twitter is constantly upgrading its infrastructure to deal with this mass of tweets. You can imagine how many bits of data need to be sent if a big star with millions of followers drops a tweet, followed by a flurry of retweets, etc. There will always be a bottleneck and a bit of clever shifting allows the data base to cope.
In the same way, when cities become bigger (we talk of metropolises and megalopolis, with more to come, I'm sure), the infrastructure needs to change with it. How to transport, water and feed millions more people in a concentrated area is a big question with our current rate of growth. For instance, the plans for a future Auckland has sparked debate on many fronts - even a whole different scheme for our transport system created by transport enthusiasts on the Auckland Transport Blog. Careful thought has to go into infrastructure alongside any future growth.
We don't want our city becoming an urban fail whale.