Why is BiH3 the strongest reducing agent amongst all the hydrides of Group 15 elements

Why is BiH₃ the strongest reducing agent amongst all the hydrides of Group 15 elements?

Answer

As we move down a group, the atomic size increases and the stability of the hydrides of
group 15 elements decreases. Since the stability of hydrides decreases on moving from
NH₃ to BiH₃, the reducing character of the hydrides increases on moving from NH₃ to
BiH₃.