There's more to it, unfortunately. As if it wasn't obvious, I've seen private studies confirming that balloon/bubble/whatever comic-like interface was carefully invented to limit people's attention timespan and granulate thoughts.
For example, on your average phone you have 3-4 short messages and TONS of empty space around. You don't have the whole line of conversation before your eyes, you have ridiculous limits to text width and height, etc.etc. which makes it irritating to read and write something longer than "Hi", forcing you to write very short messages. (I won't go deep into this, to not to be instantly labeled a conspiracy theorist, but it has enormous psychological impact, not just UX).
