Hi Terry,

This has to do with reliability. If you use a very large contact, then the metal may penetrate all the way to the active region (diffusion).
This would then result in a short. Hence, one uses multiple contacts instead. Usually, the dimensions of individual contacts and vias
are fixed for a given technology.

Razavi discusses this and other issues in the last chapter of his textbook on analog IC design.

Regards
Vivek

Terry,

  One other reason is manufacturability. If the layout has large vias and small vias mixed
in a layout. Then to resolve the small via windows, the large vias are overexposed. This
results poor control over the large via windows and makes DRC checks more complex.
The enclosure and overlap rules for via windows become highly area dependent. In addition
to photo effects, the etch control also tends to suffer. As a result, the manufacturing guys
strongly prefer layouts with a single fixed via window size.

                                                                                    Best Regards,

                                                                                       Sheldon