See for example the left button produced by this:

(x-popup-dialog t  '("This is a loooooooooooooooooooooooooooooooooooooong
question" ("Yes" . "yes") ("No" . "no")))

While the right button is just by the border of the dialog, the left one is
far from the left border.

Seems to be a maximum width for the stretching of the space between buttons.

Tested in gtk3.