Elasticity of the Cloud.
All cloud services have a page where they boast about their top users. A developer may believe that it is marketing and ignore it. That is a tad immature.
Time for a pithy one-liner :
The cloud is as big as its largest customer.
The reason is nobody builds infinitely scalable data-centers. Software runs on hardware, which has physical limits, sometimes limits like speed of light. And then there are business reasons.
Usually it is the largest customer who pushes their limits.
For example: Netflix runs on AWS, so you won't likely run into problems running a video streaming app on AWS, as Netflix would have already discovered those problems and AWS would have fixed on them.
It is symbiotic relationship, the big customer discovers flaws and good cloud providers fix them. So the next time you are deciding which cloud service to use, carefully look at their biggest customers.