Despite the increase in size of the substituent $Y$ from $F \to I$, the  proportion of $o-$ isomer increases. An increasing steric effect will, as with the  alkyl benzenes, be operating to inhibit $o-$ attack, but this must here be  outweighed by the electron-withdrawing inductive/field effect exerted by the  halogen atom $(Y)$. This effect will tend to decrease with distance from $Y$, being exerted somewhat less strongly on the distant $p-$ position compared  with the adjacent $o-$ position. Electronegative $F,$ and relatively little $o-$  attack thus takes place on $C_6 H_5F$, despite the small size of $F.$ The electron-  withdrawing effect of the halogen $(Y)$ decreases considerably from  $F$ to $I$ (the biggest change being between $F$ and $Cl$), resulting in  increasing attack at the $o-$ position despite the increasing bulk of $Y.$

$Increase\,\,in\,\,size\,\,of\,\,Y$  $\begin{gathered}
   \downarrow  \hfill \\
   \downarrow  \hfill \\
   \downarrow  \hfill \\
   \downarrow  \hfill \\ 
\end{gathered} $ $\begin{array}{*{20}{c}}
  Y&{\% \,o\, - }&{\% \,p\, - } \\ 
  F&{12}&{88} \\ 
  {Cl}&{30}&{69} \\ 
  {Br}&{37}&{62} \\ 
  I&{38}&{60} 
\end{array}$