Note: If you reinstall the Mac OS or your printing software, you may need to reset the preferred default settings.
As different printers have different physical margin limits, if you created a document that was close to the margin limits that would print on one printer, that document might get cropped when printed on another. The US Letter Small, US Legal Small, and A4 Small page sizes were developed to help avoid possible Page Layout issues.
Additional Information
A drawback to these "safe" paper sizes is that you loose about 5 mm on each margin. Nevertheless, the additional printable area is rarely used and a document formatted to fit this "small" size prints the same on any printer. Those who need to print to a larger printable area can select A4 or US Letter, and make it the default for all print jobs.
Prior to the creation of US Letter Small and A4 Small, the printable area for US Letter and A4 was in fact the same as for US Letter Small and A4 Small. Some customers, however, selected US Letter or A4 in conjunction with an option "Larger Print Area" that gave them larger margins, and this size is now the same size as the present "US Letter" or "A4" sizes.
Selecting US Letter or A4 requires more printer memory for an area of margin that is rarely used when printing standard business documents. This causes slower print times as well as prevents some complex documents from printing if the printer runs out of memory. Therefore, the default is (and has always been) the smaller margin area.
Note: Paper size options are obtained from the printer's PPD. If a particular paper size is not listed in Page Setup, for instance US Letter or US Legal Small, it may not be an available option for that printer.