Why is blowout often bigger than sag?
Sag is always the no wind mid span sag (unless specifically defined to be something else). Blowout is the horizontal component of the conductor swing under the blowout temperature and wind pressure conditions, which will (nearly) always be bigger than the no wind sag.
Looking at this figure from AS7000:2016 (ignoring insulator swing) you can see blowout (y).
See https://ipowermation.com/articles/conductor-blowout/ for more comments on blowout.