As stated in this answer, an elliptical speech bubble is more commonly seen than a rounded (rectangular) speech bubble. This is perhaps true of comics, but not in messaging/chat services; it makes sense that you wouldn't want a round speech bubble for long lines of text.
On the other hand, ellipses are more popular for the actual icons, as seen with Messenger, Hangouts, iMessage, and WeChat, among others:
Then there's stuff like this one:
These rounded-but-not-elliptical-speech-bubble icons seem much rarer (along with other non-elliptical icons), even though all of these above services use rectangular speech bubbles, and many interfaces use rounded icon masks instead of elliptical ones.
We seem to have a lot of thoughts about sharp or round edges but are there any studies about usability between fully rounded speech bubbles vs. partially rounded speech bubbles in terms of icons for messaging? Are there perhaps implicit usage associations, or is it simply that elliptical speech bubbles seem more "friendly" shape-wise?
For bonus points, are there any studies about whether there should be one, two, or more speech bubbles in the icon, and whether they should be blank or not? Should the tail be straight or curved, the tip pointed or rounded? Why not point to the right? There don't appear to be many sources on the iconography concerning messaging.