The field of Democratic candidates is being belittled by commentators as being way too big; they're "nine dwarves", etc.

Lest we forget, there were ten Republican candidates for the 2000 Presidential nomination:

Lamar Alexander
Gary Bauer
George Bush
Elizabeth Dole
Steve Forbes
Orrin Hatch
John Kasich
Alan Keyes
John McCain
Dan Quayle
(from Washingtonpost.com: White House 2000)

...and against that field of challengers, the establishment choice prevailed and succeeded in occupying the White House.

So how does the presence of nine Democrats make this a silly endeavour? How does it imply that all of them are unworthy of the office?

It doesn't. Reporters, columnists and especially webloggers just like to belittle things.