I agree, move the writeToFileNamed outside the loop. Instead of adding each file to 1 zip, you are doing something like the following:

• Zip up file 1
• Zip up file 1 and 2
• Zip up file 1, 2, and 3
• Zip up file 1, 2, 3, and 4

You should add all the files, then writeToFileNamed at the end.