What is your MongoDB version == > 4.0.14-8
This seems to be a version of the Percona fork of MongoDB server. Is this correct?
If yes, you might want to contact Percona support regarding this behaviour, since it involves modified server code that may work differently from official MongoDB servers.
What I did notice from your screenshot is, if your collections are so large, the index sizes are suspiciously small. This could be either 1) the large collections are not indexed, or 2) the Percona fork of the server is behaving differently.
does size of collections also impacts performance
In most cases, yes. If improperly indexed, larger collections would require more resources to process. For example, a query with a collection scan would then require the server to load all 8.5/60.7 GB of data from disk into memory to answer the query. This will be a massive workload if the server was provisioned with a smaller amount of RAM and slower disks.
Having said that, if your working set (data that’s frequently accessed + indexes) can comfortably fit in RAM, performance impact could be minimal even with large collections.