Is twitter's character limit a way they found to avoid drastic database fragmentation?
The original character limit in twitter was because it started out as a micro-blogging platform with the option to update via SMS. SMS messages had a limit of 160 bytes. The 140 limit on the message length is because the extra metadata (to identify the user) was up to 20 characters long.
It had nothing to do with things that could affect internal or external fragmentation (rows/page, dynamic/static row sizes, excess page splitting due to key values effectively in random order, ...) or other concerns of the backend database design.