All other things being equal, more development = more contrast and more graininess. "More" development can be accomplished in several ways. The easiest is to keep everything constant and just extend development time. However to some extent increased agitation also gives you more development, meaning you can theoretically get more development by keeping the time constant and agitating more frequently (or agitating constantly).
So basically if you hold all other variables constant, you can get higher contrast and graininess by extending development time, or increasing agitation or a combination of both.
Just remember the increase in graininess we're talking about here relates to the amount of development taking place, so the relationship between agitation and graininess is indirect. Agitation, in and of itself does not influence graininess. Agitation affects the amount of development taking place in a given amount of time. And more development = increased graininess.