Recently I had to work on a file provided by a client. The file was more than 70MB in size. Reducing the size was not part of the customer's request but my internal optimizer kicked in and after some investigation I found unlikely cause for this horrendous size.
I started with the usual suspects in the file size - doubled data and calculations, excessively complex charts, pics, some embedded elements and some others. Not one of them proved to cause the problem. After some trials it hit me that ranges with data are heavily formatted and pasted from other files as is. I removed the formatting on the data sheets and to my surprise the file size shrank to 15MB! I have never suspected that the formatting has such a huge impact on size. True, there were about 20 sheets with data spread over 1000 rows and 100 columns on average but still but still surprising.
The old proverb holds true - as long as you there is something new to learn! :)