Pressure is inversely proportional to Area as P=F/A where P is Pressure, F is Force and A is Area. When we stand on one leg our less body surface area is in contact with the sand. As a result, Pressure on sand increases and thus the force applied also increases. So, we sink more in sand while on one leg.
Similarly, we apply more force on the ground while walking than that we apply when we are standing. Please ask me for further help if needed.