Why is the difference between the melting point and boiling point large for some compounds, and small for others?
well thinking that the same factors influence the melting and boiling point of materials is possibly wrong.
high melting point is due to strong packing(highly symmetric molecules) while  high boiling point is due to large surface area
so if a compound has good packing and less surface area will end up to have very less difference in melting and boiling point and sublime