Size matters, or at least as far as the photographic print market is concerned it matters. The same is largely true of the art market in general, larger pieces tend to sell for more than smaller pieces. Whether or not this is justified or even desirable is a matter of opinion and conjecture.
Personally speaking I can't help feeling that pricing by girth is rather bizarre but it is what the market expects. Some of my favourite image makers only offer prints sizes that I would describe as being 'intimate'.
As has been mentioned elsewhere there are other factors that influence price, sales outlet, market, open or limited edition, size of edition, to mention but a few. On a personal level the decision to limit the edition or not depends on the intended market, potential sales and also to a certain extent on vanity. There are images that just scream to be sold as open editions, and others with perhaps a rather limited market that lend themselves to being sold as limited editions. If I know an image will sell time and again and will sell over a period of many years then where is the sense in severely limiting those sales? Conversely, if I know an image will appeal only to a highly targeted audience then limiting the edition and increasing the price makes sense. Vanity?...well, there's a part of me that thinks I'd rather sell more prints to more people rather than fewer prints to fewer people.
Having said all of this, and having sold images - prints and image licensing - ranging in size from thumbnails to 84" long side, it's rather perverse that the best earners have probably been smaller sized open edition prints and web use thumbnail licenses that are renewable yearly.