Theoretically, there is no maximum object size in Atmos.  Overhead is a little less in Atmos, but not much.  If you already have bundling baked into your application, I would keep it around to be on the safe side.  If your objects vary greatly in size, you can also establish policies based on object size to help efficiency.  For example, you can have objects larger than 512k use erasure coding (parity) to reduce overhead.  This is covered on page 195 of the Administrator's Guide.  FYI, when ViPR Data Services is available (which supports the Atmos API), it automatically bundles small objects on the server side, completely transparent to the user.
We are currently using the Centera, and we have need to zip files for two cases:  Moving forward to Atmos, what strategies should we consider for optimally storing files?  Is it fine to store many small files?  What is the maximum file size?
What are optimal file sizes?