Why does the trimethylammonium cation have a larger -I effect than the ammonium cation?
$\ce{N(CH3)3+}$ has more -I than $\ce{NH3+}$
, this irony happens because if you consider
$\ce{N-CH3}$ bond and $\ce{N-H}$ bond,  which is more polar?
Of course $\ce{N-H}$ bond will be more polar  due to more difference in electronegativity  value,  so that implies electron density will be more on $\ce{N}$ in $\ce{NH3+}$ than $\ce{N}$ on $\ce{N(CH3)3+}$.
And therefore since electron density is less on $\ce{N}$ of $\ce{N(CH3)3+}$ , it will have greater -I effect.