A quick workaround for this problem I've found is to:
1. Click the paperclip icon to attach a file.
2. Choose your file and hit insert.
3. Right click on the giant icon and select Edit Media.
4. For dimensions, enter 75x75 (or 25x25 for a really small icon) and click insert.
5. The icon should resize.
Similarly, you can use the Insert/Edit Media button instead of the paperclip button. Using that, you can enter the 75x75 (or 25x25) dimensions right away and not have to edit the huge icon.