Why bulky group present at axial position of chair or boat conformer is less stable compared to equitorial position? 

Ans) Let the bulky group has a large mass M.

suppose it is at the axial point at a distance r from the centre.

then the torque apllied is=r.M.g

hence torque is proportional to the distance fom the centre

larger the distance more the torque and less is the stability of boat or chair.