Well the first dlc will not be the size of shivering isles because it hasnt been near long enough for them to make something of that size..
your working under the impression that they haven't already built it, im not saying they have, but if you take COD for example...purely example, the dlc's for it are already built in, all the maps are there ready to go there just locked down until there bought and unlocked, i generally work under the assumption that Bethesda has learned from the success and demand of brilliant DLC's from Morrowind, Oblivion, Fallout 3 and F:NV and started the DLC's long before, prolly some time around release...but you know what they say about assumptions just my 2cents