Large difference in boiling points is observed in

The size of element increases as on moving down the group. Sb and Bi belong to the same group but the variation of boiling point less in lower elements of a group due to inert pair effect.

Inert pair effect is weaker from N till As, but it is maximum in case of Bi and hence the electrons in s orbital remain unbounded and the melting point or boiling point decreases because of decrease in interaction. So, a high difference in the boiling point observed from As to Sb.