g (9.8 m/s^2) is an acceleration, not a speed. It's right there in the units. The actual terminal velocity of an object is meet when its wind resistance -- which is a force (acceleration with mass) with negative magnitude -- matches g. Until then, it will accelerate.

So, generally, the larger the surface-area-to-mass ratio of an object, the lower its terminal velocity. Aerodynamics also plays a part, of course. So, a lead feather will have a much higher terminal velocity than a real feather, due to the extra mass that the wind resistance is fighting against. Meanwhile, g is an acceleration so it affects all mass equally.

Here's a really cool video of a feather and bowling ball falling in vacuum together:

https://youtu.be/frZ9dN_ATew

Since aircraft rely on lift, which is also a force, to overcome gravity, minimizing weight is ideal. Gravity will affect all the mass equally. But the same lift force with double the mass will only be half as effective.