Preview's "Reduce File Size" feature takes a 512KB file down to 254KB, but makes the text very blurry and difficult to read.
On Acrobat Standard (Windows), the "Reduce File size..." feature takes the 512KB file down to 418KB, but maintains the exact same quality that the original had.
Therefore, reducing the file size after the file was combined is not a good option. A better option is to use a more efficient program to combine the files...
I found out that Acrobat Standard is better than Preview at combining PDF files and maintaining a small file size. For example, if you have two 61KB files to combine, Preview will produce a 311KB file, while Acrobat (with the "Default File Size" option enabled) will produce a 106KB file.