An atom is made up of equal no of protons and electrons when an atom loses an electron it becomes cation.therefore the number of protons is more in the cation. So the positive force attracts the negative cloud of electrons thus decreasing its size.
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Cations are deficient in electrons. so the protons exert greater force of attraction on the electrons thus pulling them closer. due to this strong force of attraction ,the cations tend to contract in size.  
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