If you see Wolff's avatar size have 3 variations.The more the lines in the message, the larger the is size. Am I correct? Or is this a bug or does it indicate something else?
I see only two variations (there's an even larger image when you hover over them, or visit the user's chat profile), but this is intentional, and it indeed depends on the length of the messages; posting multiple short messages (without interruption) also causes the avatar to be displayed in a larger size.
I could not find a place where this is documented, but here are some related bug reports on the main Meta:
The second post has a status-bydesign tag so this behaviour is definitely intentional.