Scaling up is a term used for achieving scalability by adding more resources to the same machine. It includes the addition of more memory or processors with higher speed or simply the migration of applications to a more powerful macOS.
With upgrades in hardware, there is a limit as to how you can scale the machine. It is more likely that you are just shifting the bottleneck rather than solving the real problem of improving scalability. If you add more processors to the machine, you might shift the bottleneck to memory. Processing power does not increase the performance of your system linearly. At a certain point, the performance of a system stabilizes even if you add more processing capacity. Another aspect of scaling up is that since only one machine is serving all the requests, it becomes a single point of failure as well.
In summary, scaling vertically is easy since it involves no code changes; however, it is quite an expensive technique. Stack Overflow is one of those rare examples of a .NET-based system that is scaled vertically.