When exporting a Flash gallery out of Photoshop Lightroom 1.0 it exports three rendition sizes of each image. Those three sizes are "relative" to the "preview size" setting, which you could kind of think of as setting the size for the "largest" of the three.
When the gallery is loaded into a browser, the gallery automatically detects the size of the window, looks at the rendition sizes available, and then downloads the largest rendition size that will fit into the window. It then does some pretty sophisticated pre-fetching of the other images of that size based on visitor behavior (linear vs non-linear browsing of the gallery). The gallery does not download the other renditions at all - unless the end user changes the size of their browser window such that a different rendition size should be used.
The gallery does not real-time scale the images inside Flash - except in the case where the browser window is made too small for even the smallest rendition, in which case it will scale down the image in real time. In early Beta versions, all the images scaled down in real time in Flash, in Beta 4, the gallery threw scroll bars on the images themselves if the window was too small. I think the 1.0 approach (swapping renditions) is much better. What do you think?
Is all this complexity overkill? perhaps. But when I look at galleries like this (awesome) one at Computer-Darkroom in a maximized window on my hi-res monitor, I really appreciate the multiple rendition approach.
Granted this takes up extra room on the web server, (pretty cheap these days though,) but if there were only one rendition, it should probably be a size that will fit on 800x600 monitors which is still well over 10% of web users, which would mean it would need to be pretty dang small given OS chrome, browser chrome, headers, etc - and then if you had thumnails across the bottom...
However, if you want to have only one rendition, here's what you do:
first open up the group.xml file in a text editor and delete mentions of the the sizes in the <sizes> node that you don't want - the whole line of each. Now you can delete the associated images from the bin folder.