Extracts take up most of the space

Workbooks by themselves are fairly small files. When extracts are used the data gets rolled in with the workbook when it is published and it can cause workbook sizes to increase. Aggregating extracts can help reduce filesize and increase performance. 

Filtering data  as part of the extract will remove unneeded data/size and Removing unused columns reduces the size of an extract. 

If the same data is used in multiple workbooks using a shared datasource makes sense as only one copy of the data is extracted onto the server. It also speeds up development and publishing.