Flash streams on load… meaning if you had no preloader it would load frame 1, then 2 and so on and you would experience lag if you caught up to the stream as waited for it to continue. Now no flash site is really too big - IF!! it is cut into sections that are all seperate swf’s that load on demand. Let me explain… say you have a site with a product, gallery, abd about section… well if you put all these into one swf and preload it, the user will have to wait for EVERY section to load… even if the only part they want to see is “about”. Instead make your site layout, and nav menu one swf and preload only that (which all users need) then creat your seperate sections as seperate swf and have them loaded (and preloaded if needed) into your main movie using the loadMovie command. That way users only have to wait for the parts they want to see load. I hope that was clear… I tend to explain things in a round about manner.
Peace