Will the smart guy that did this show you how? If so, go and see what they do, and what version of which software they used. Changes from version to version, not always for the best.
Otherwise, you can try some detective work. Use the Audit Space function from the PDF optimizer on the 8M and the 2M file. Let us know what you see. Also look under File > Properties and see if "Creator" or "Producer" has changed.
Also note that you cannot legislate the size of a PDF file. How much a file can be reduced in size, if at all, depends upon the content (text and vector graphics typically can't be significantly reduced in size) and the degree to which you are willing to lose quality (such as with raster graphics - lower resolution and lossier compression can cut PDF file size but the results may be effectively unusable).