They're storing all the metadata, and for better or worse they are storing each string field as an "object" whereas they could have used a fixed-width field. The former of course is something like a reference count and a pointer to a string elsewhere; the latter would have some overhead when strings are of varying lengths. If memory usage were really a concern, decent savings could be realized by changing (most of?) the 13 float64 columns to float32, and some of the object (string) columns to fixed-width. For example, "lang" is two characters always, and usernames (which are stored in several fields) would be more efficiently stored as char[15]. Easy to try, just not the default behavior; I bet you could cut the total size by a third (too bad they didn't).

Tweets come with a ton of metadata these days.