Sorry to hear you're having issues with my changes. Just to justify myself a little bit: the old way of generating the radius field silently produced incorrect results on non-periodic datasets. yt is increasingly being used to examine these datasets and I wanted to make sure that the results of a simple radial profile analysis would be correct.
That being said, the memory consumption is clearly not as good now. When I wrote the new code to handle the radius fields, I didn't realize that the Radius field was so intimately tied to halo finding.
A quick fix to allow you to continue doing your analyses would be to replace the Radius field with the old definition. This can be done without committing any changes to the yt codebase by using the my_plugins.py file. Just create a file called my_plugins.py, place it in the .yt folder that lives in your home directory, and enter the old Radius and ParticleRadius field definitions inside of it. I've pasted an example my_plugins.py file that does this here: http://paste.yt-project.org/show/3212/
Fields defined in my_plugins.py will override definitions in universal_fields.py, so if the new field definitions are the primary cause of the increased memory consumption you're seeing, this should fix it.
This isn't a very good long term solution and I'd like to work with you and others who deal with large datasets to find a permanent solution. A good first step would be to file an issue about this. I don't have a lot of experience working with large datasets so any help figuring out how to reduce the memory needs of the Radius field would be appreciated.