on moving down the group the atomic size as well as nuclear charge inreases.But the effect of increase in atomic size is much more pronounced than that of nuclear charge and thus the additional electrons feels less attraction consequently electron gain enthalpy becomes less negative on going down the group
X (g) + e- ===> X- (g)
F = -328 kJ/mol
Cl = -349 kJ/mol
Br = -325 kJ/mol
I = -295 kJ/mol
However the electron gain enthalpy of F is less negative than of Cl because of its small size. The additional electron in case of F will be added to the 2p orbital while in case of Cl it will go in the 3p orbital. As a 2p orbital occupies a smaller region of space than 3p orbital, it will be more compact because of which electron-electron repulsions would be more. Therefore the incoming electron would be accepted with much less ease in case of F. Hence its electron gain enthalpy is less negative than Cl.