Here you can adjust the overall photo and sound compression.
The lower the compression quality, the smaller the published file.
Note that when you check Compress photo, the published quality settings in this dialog box will be used to compress all images (background and non background image) during publication. Image compression settings will not be modified but instead it will not get used during publication. However, the original image compression quality will be used instead if it is lower than the published compression quality.
Same rule is applied to Sound compression.