Shared workbook keeps increasing in the data back to the shared book

[I_know_nuffin]

Hi All
I have a shared workbook that other staff update - its shared so multiple staff can work on it at the same time

Every morning I run a report that copies the data from 1 tab in the shared sheet into my reporting workbook
this is all done via a VBA routine
open shared book, copy data, paste into reporting workbook, close shared book, the bit I don't understand is why the shared workbook increases in size by about 100k when it closes

Once I have run the days reports I copy the results back into the shared book
open shared book, empty data TAB, copy data, paste into shared workbook, save and close shared book, the other bit I don't understand is why the shared workbook now increases in size by about 300k

Can anybody shed any light on why the size keeps increasing, and suggest how to stop it

Thanks

David