Bitbake downloads the source as it goes along - it downloads the source required for a single package, then builds and packages it, then moves onto the next.
35GB is not required for a bitbake opie-image - for a bitbake world perhaps.
If you just want to download the source, you can tell bitbake to just perform the download step. The exact command escapes me (look on the OE mailing list) but it's something along the lines of:
bitbake -c fetch opie-image
Si