In series circuits, the same current flows through the circuit.

The resistor with the greatest resistance has the greatest voltage drop. According to ohms law, we know ΔV = IR

Since the current in the circuit is the same in series circuit, the I is the constant for the circuit. Hence, by ohms law, the potential drop varies according to the resistance in the circuit. The potential drop varies according to the varying resistances in the circuit, as the current flowing through the circuit is the same.