> There is no reason - beside manually managed infrastructure - to not decrease the instance size (number of machines or c3.xlarge to c3.large) if you realize that your EC2 instances are underutilized.
CPU/Memory aren't the only measures of underutilization. If you require high instantaneous bandwidth throughput, then the networking capacity available to your instance roughly increases with the size of your instance. This includes both EBS as well as other Network traffic.
Table with Low/Medium/High: https://aws.amazon.com/ec2/instance-types/#instance-type-mat...
Example benchmark with c3 instances: http://blog.flux7.com/blogs/benchmarks/benchmarking-network-...
If you're more concerned with just EBS network throughput, check out the table on this page instead: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-ec2-...