My guess is that the bubble just calculates the required margin at both sides of the word:
--word--
--longerword--
What happened in this case is that, because the letters of the typeface have different widths, it looks like the bubble is staying the same size. When, in reality, it's just a coincidence. If the font were fixed-width, so each character measured the same horizontally, then you would notice the difference in length:
--iii--
--iiiii--
--mmm--
The same with a non-fixed-width font:
--iii--
--mmm--
(both have 3 characters, one uses more space).