The goal is to minimize deflection. There are two components of that, along the joist and for the flooring, inbetween the joists. It depends on the strength of the flooring material you choose whether you can extend the spacing. You wouldn't want to stand in the middle of the joists and have the flooring sag. this is especially important when using one of the man-made materials like Trex (just one example). Thicker materials can support larger spans, both for the joists and the floor. Doubling the joists and using a double joist hanger may be less expensive than the thicker material. You wouldn't want to set something heavy inbetween a large joist spacing and have it sag, say a big barbeque or the weighted base of a table umbrella.