A large guest list means more tables, linens, chairs, flatware, etc.
May, June, September, and October are the peak wedding months. Prices are higher then because of the greater demand and availability may be limited as well.
Not only does the distance traveled to your venue factor into the overall cost, but the ease of accessibility at the site itself also matters. If the rental items need to be carried a great distance (e.g., across a bridge, through the woods, etc.) instead of simply unloaded straight from the truck, the added time and labor will add to the expense.
The level of intricacy also influences the price. If you have a more casual affair and need less rental items, this will cost less than an upscale event with numerous rentals.