Well, it's important to remember that Windows Vista, 7, etc are image-based formats (well, a file container as an image, technically), and any service pack is going to include quite a few of the files that exist in the base install (because it updates them). This means a service pack for something like Vista or 7 (or the server variants) is going to be almost as large as the base install itself. If installing SP1 to an existing Windows install, you can run a dism command (dism /online /cleanup-image /spsuperseded) to remove the Side-By-Side caches of pre-SP1 files after you install SP1 to make sure you're not keeping around pre-SP1 files in that existing installation, which will reduce installed footprint by a bit after the command is completed.
When you slipstream the SP, however, your base WIM is likely to grow by quite a bit because the slipstream is a file replacement rather than update. This may not make sense at first to those new to the WIM format, because you'd think replacing files from RTM with a service pack should really not grow much at all. However, the WIM container will grow a bit with each file replaced because replacing a file in the WIM requires the new file to be inserted, and the old file removed. The WIM container format itself cannot "defrag" itself when you increase the size to add or remove (or both) a file, even if you're adding an update to the same file (as in a service pack). The addition of a file causes the WIM to grow, and the removal of a file causes "white space" the size of that file (or thereabouts) to be left in the WIM container. Thus, modifying an image in a WIM container causes it to grow any time you add or remove a file. What you need to do in this case is to "compress" the WIM after you've finished modifying it. First, you would slipstream the SP into each version that exists in your WIM file, and then you would export (using DISM or another tool like gimagex) each version into a new WIM file. This will "compress" the WIM back down in size, because (as I mentioned in the beginning of my post) the WIM image format is really a growable single-instance-store file container. The only way to "compress" a WIM once you've made changes to it is to export it from it's existing (fragmented) container WIM into a new container WIM (which causes all of the files inside to be copied to the new WIM container sequentially, thus removing the "white space").
