As Max mentions, the Reduce Size PDF command is really just one particular setting of the PDF Optimizer that is available in Acrobat Std. and Pro. In Acrobat XI, it is located under the File > Save As Other > Reduced Size PDF menu command and can be run on multiple PDF files. It will resample and compresses images and subset-embeds fonts that were left embedded. It also compresses document structure and cleans up elements such as invalid bookmarks. If your file size is already as small as possible, the Reduced File Size command will have no effect on the size of your document. Remember to not perform this command on digitally signed documents.
On the other hand, the PDF Optimizer (only available in the Pro. version), allows you to pick and choose what elements you can remove and modify in your PDF. This command is located under the File > Save As Other > Optimized PDF menu in Acrobat XI Pro.
In the PDF Optimizer you can do things like downsample and compress images, unembed fonts, work with transparency, flatten form fields, and remove links and bookmarks.