How do I make Udon noodles fat?
Many of the dried noodles that are marketed as "udon"—at least in my experience in the USA—are actually mislabeled, thinner noodles like Hiyamugi or even Sōmen.  I would suggest buying the semi-dried variety that are usually packaged in vacuum sealed plastic.  This variety is shelf stable, but it can often be found in the refrigerated section of Asian mega-marts.  The noodles will already be almost full size before cooking.