Did you try decreasing the left and right margins?

I seriously doubt that the printer driver knows anything at all about the source material. It is almost certainly just taking the page data sent to it, shrinking it, and reordering it.

If you take an image of an 8.5" x 11" page and shrink it so that its width is 5.5", the height of the resultant image is 7.12", which is over an inch shorter than the paper you're printing on. Add in your margins, and it's not surprising that you're getting so much wasted space.

If you're concerned about this, find some A4 paper and use that instead. I think Mexico is technically standardized on the A?/B? paper sizes, so you ought to be able to find some. Alternately, you might be able to finagle something by specifying a weird paper size. You're trying to get your pages to be 5.5" x 8.5", so maybe tell Excel that your paper is 8.5" x 13.14" (the same aspect ratio)?