It may happen that Sparrow for Mac uses an excessive amount of disk space. For only the headers of my 9GB of e-mail, Sparrow used 39GB. I found a work-around.
The problem
Sparrow uses a Tokyo Cabinet, called data.tch, as storage. Mine was 39GB. What could eat up all that space? I only have 9GB of e-mail in total. With a simple script I looked into the cabinet and found it actually contained less than 1GB of data. The cabinet was wasting a lot of space by not being packed tightly.
The work-around
To repack the Tokyo Cabinet, run:
tchmgr optimize "~/Library/Containers/com.sparrowmailapp.sparrow/Data/Library/Application Support/Sparrow/(your-email@gmail.com).sparrowdb/data.db/data.tch"
Replace (your-email@gmail.com) with your e-mail address. You can install the tchmgr program using sudo port install tokyocabinet, if you are a MacPorts user.
This reduced the size of the cabinet to 1.1GB!
Cause?
It is odd that the cabinet became so loosely packed. It may happen if Sparrow adds and removes a lot of entries, but there seems no reason for Sparrow to do this. There might be an underlying bug. I reported this issue to the developers, but they have yet to respond.