Investment multiplier and MPC are directly related to each other. It can be as follows:
$\mathrm{K}=\frac{1}{1-\mathrm{MPC}}$
If $\mathrm{MPC}=0.4$
then$\mathrm{K}=\frac{1}{1-0.4}=\frac{1}{0.6}$
If $\mathrm{MPC}=0.6$
then$\mathrm{K}=\frac{1}{1-0.6}=\frac{1}{0.4}$
$\therefore$ Both are directly related with the increase in MPC, investment multiplier will increases.