Which is a stronger reducing agent, SbH₃ or BiH₃, and why?

Bi lies below Sb in the periodic table in the same group (15). Due to which, the size of Bi is more than that of Sb.

Now reducing agent is one which can readily produce hydrogen. As, Bi is large in size, due to lower nuclear attractions the Bi-H bond breaks easily. As a result H⁺ is produced easily. So, it is a stronger reducing agent than SbH₃.