Have you ever noticed when you edit your WIM files they tend to grow considerably in size even if the file you just copied over was only 2MB in size. In order to reduce size of the image you can run imageX /export + parameters to reduce the size.
imagex /export d:wimfilesorig.wim d:wimfilesNew.wim 1
This will take the existing WIM file and create another one, and in the process it will remove unnecessary references and resource files needed.
Another option if size matters to you is to include the /compress maximum. Just remember that if you want to use one WIM file for several images they must all use the same compression level. Default compression level is Fast.
More on the subject: http://technet.microsoft.com/en-us/library/cc749447(WS.10).aspx