Hi Vishakha

Size of cation is smaller than the related atom .

Example - 

Na = Proton 11, electron 11 (Equal force of attraction) 

Now if sodium looses its electron, it get converted in 

Na⁺ = Still 11 proton but 10 electron

Now These 11 proton will attract 10 electron ( the positive charge are more than negative)  

Thats decreases the atomic radius.

Hope you got your Answer

Thanks for asking