Hi,
One option is to disable the sprite controls - (preferences, enable sprite controls). The handles are controlled by that.
If you want them enabled, but want it to be the tighter bounds, tk2dsprite.cs, line 249 or so
void OnDrawGizmos() {
Gizmos.DrawCube(Vector3.Scale(sprite.untrimmedBoundsData[0], _scale), Vector3.Scale(sprite.untrimmedBoundsData[1], _scale));
Rename untrimmedBoundsData to boundsData