Excel docs become HUGE when I save them

I am a rookie mac user, and am required to use excel because most my co-workers are on PCs. Every time someone sends me a somewhat large excel doc (3MB) when I open it, it:

1. takes forever

2. the file size jumps from 3MB to 50MB!

3. If I try and save it, it will save at the massive size

4. If I try and copy the content into a new worksheet, it still somehow pulls over the massive size and saves as 50MB.

Help. I have no clue why, or what to do! Half the time it ends up causing excel to just flat out shut down.