If you did the same experiment where there is no matter or simply we say 'vacuum', you'll see that no matter what their mass is they'll fall at same speed and will reach ground at the same time, whereas this case is completely different practically.
The reason behind is 'drag force' or 'air resistance'.
More the matter is less will be drag force and less the matter is more will be the drag force.
This drag force reduces the gravitational force exerted by earth and hence its downward acceleration decreases. As a result lighter object takes more time to reach ground than heavier objects.
Hope this helps!
