Let's start with a basic distinction: what is the difference between Scaling Up and Scaling Out ( or between Scaling Vertically and Scaling Horizontally)?

Scaling UP means adding more resources to an instance.

Like for example we could scale up from a T2 Instance with 1 vCPU and 1 GB Ram to a C5 that has 8 Gb RaM and 4 vCPU.

Still, single point of failure

Scaling OUT (also scaling horizontally) means just adding more instances.

provide more resiliency, if one instance fails, you still have other running.

Scaling IN means simply reducing the number of instances, whenever the need is decreased.