|Here are a few things to try:|
In your formulas check that you have used only the range necessary,
NOT the whole Row or Column.
If your in the habit of using something like A:A in ranges
then your using 1,048,576 rows. That's a lot of processing.
If you have Conditional Formatting, again, check to make sure you are using
only those Row's and Columns necessary and there are no stray formatting
You could also Copy only the used cells into a new worksheet.
Some files may be bigger than necessary due to extra cells without any content being included in the worksheet