How did Jiraiya find out Tsunade's breast size? Jiraiya stated that Tsunade's oppai size was 106cm.

From Tsunade article:


  Despite being known as flat-chested in her childhood, she now has a rather large bust; 106 centimetres in circumference, according to Jiraiya.


From Jiraiya article: 


  when caught spying on Tsunade in his younger years, she proceeded to break both of his arms, six of his ribs, and ruptured a number of organs as punishment.


Considering the amount of fear and respect Jiraiya then developed towards her, how did he know the measurement of the adult Tsunade?

Source: https://www.youtube.com/watch?v=ny7Fni193Jk

(I'm really sorry about the quality of the video)

Below is the image from the corresponding scene in the manga (chapter 406, page 9):
Maybe Jiraiya transformed into Tsunade using a transformation technique and found the size using traditional measures by measuring himself(!).

He may have used Sexy Technique.