When creating WinPE 2.0 boot images, make sure you run the PEImg /prep command against your boot.wim as it does make a significant difference in sze as shown in the table below:
|Bootable ISO file with WiinPE 2.0||Size in MB|
|boot.wim prepped||146 mb|
|boot.wim not prepped||203 MB|
Considering this all is loaded into memory, you might want to make sure to reduce the size of the boot.wim to an absolute minimum.