Given a list of [FileName, FileSize, [Collection]] - Collection is optional, i.e., a collection can have 1 or more files. Same file can be a part of more than 1 collection.
How would you design a system

To calculate total size of files processed.
To calculate Top K collections based on size.
Example:
file1.txt(size: 100)
file2.txt(size: 200) in collection "collection1"
file3.txt(size: 200) in collection "collection1"
file4.txt(size: 300) in collection "collection2"
file5.txt(size: 100)
Output:

Total size of files processed: 900
Top 2 collections:

collection1 : 400
collection2 : 300