basic

well, F is much smaller than Cl and i.e. the relative difference in their radii is about 38%.due to this,an e^- can't enter into the orbitals of F to form the -ve ion more comfortably than Cl.
in other words,its due to relatively greater effectiveness of 2p-electrons in small F-atom to repel the additional electron entering the atom than 3p-orbitals in Cl atom

the reasons are :
(1) mutual electron repulsion in the small fluorine atom.

(2) d∩ - p∩ backbonding is possible for chlorine atom!!!