What does 'Save as > Reduced size PDF' do compared to 'Save as > Optimised PDF'?

(Acrobat X Pro)

So, both "Save as > Reduced size PDF" and "Save as > Optimised PDF" allow you to reduce the file size of a PDF. "Optimise" offers a variety of options, "Reduce" just gets on with it.

What is the difference between what they actually do under the hood? Does "Reduce" essentially run the "Optimise" process with certain default settings? Or is it a wholly different process?

What effect does "Reduce" have in terms of what elements of the PDF are compressed and removed?

In my experience, "Reduce" seems to be less prone to crashing, and often does as good a job as "optimise". It preserves things like hyperlinks, which optimise may or may not do depending on settings. I'd like to know more about what is actually happening under the hood so I can better judge which is the best tool under what circumstances.