Overview GOJS highlight box is taking full box

At the time of orangebox:
Overview.scale is 0.0005398244706751153 and
Overview.viewportBounds is
{x: -555, y: -20.000000000000227, width: 1852.4540000000002, height: 1852.4540000000002, s: true}

And Late it is :

Overview.scale is 0.03832753741793318 and
Overview.viewportBounds is
{x: -555, y: -20.000000000000227, width: 3156.999070422535, height: 1852.4540000000002, s: true}