If you do nothing special then most mobile devices will just scale down the page to fit on the screen which is why the site will look very very small. You then have to pinch and zoom to see the detail. The iphone assumes a screen width of 980px so if you have designed for 980px it scales it down reasonably well to fit 320px but if the original site is bigger then it won't fit quite so nicely.
If you don't want your site to be squashed small then you need to tell the device what you want it to do and this will involve setting meta tags in the head of the page to control the viewport size and then use media queries to completely change your content into a form suitable to fit the device - usually a one column simple layout.
It's not something you can do in a couple of seconds and usually has to be built into the design from the start for best results.
More info here.