This type of question often leads to a good deal of confused thinking. Here is an explanation I use with my own students
What actually happens is by no means obvious, so I'm going to resort to a little math, just to maintain clarity and credibility
Let's consider two aircraft that are externally identical in every way. However, one aircraft is much heavier than the other. They both have exactly the same thrust and drag. Now we will assume that they both dive from some initial speed and we want to know what happens next. But firstly, let's pause to consider the average layman has to say about it
"I thought Galileo proved that all objects fall with the same acceleration?"
Ok what happened is this... He noticed that he could drop metal balls of different weights, from a building and they would hit the ground at the same time, at least he couldn't measure the difference. This is how the math clocks out. Using Newton's law f = ma where f is the force acting on a ball, m is its mass and a is its acceleration towards the ground. We are interested in how the objects accelerates so we write it like this:
a = f/m (1)
That simply says that the acceleration is the force pulling the balls down to the ground divided by their mass. Now if you consider the drag acting on the balls to be negligible, the only force left to consider is the force due to gravity, so we get:
f = mg (2)
Now if you substitute that value for f back into our first expression (1) we get:
a = mg/m
The mass m cancels out leaving:
a = g and g is about 9.8 m/s^2 or 32.2 ft/s^2
This means that if we can ignore drag (which is approximately true for balls that are heavy and smooth) the balls will accelerate at the same rate, regardless of their mass!! Certainly for casual observations that appears to be true.
However, the situation is very different if we consider an aircraft where the drag and thrust can't be ignored, because they are no longer negligible.
So let's consider our aircraft, but firstly let's just consider the drag, and we can call it d for short. Now we can still write Newton's law just as we did in expression (1) but now when we write down the force acting on the aircraft we need to include the drag too, so we get:
f = mg - d (3)
We write a negative sign because drag always opposes the motion. Now look what happens if we substitute the value for the force in expression (3) back into expression (1), we get:
a = mg/m - d/m
a = g - d/m (4)
Now here is the crunch! The mass doesn't cancel out of the drag term, so the mass will now have an influence on the acceleration. But what influence?
Ok, look at expression (4) again, it is saying that the acceleration is going to be reduced by the drag (yep that makes sense) but as the mass gets larger, the actual value of d/m will get smaller, meaning that for the same amount of drag, heavier objects will accelerate more quickly!!
However, it causes no end of problems because our everyday experience tells us that heavy objects are more difficult to accelerate than lighter ones.
Now take another look at expression (4)... The acceleration is maximum when the d/m term is as small as possible, which means that for the best acceleration in a dive you want a combination of low drag and high weight, which means that the smallest drag/weight ratio possible will give you the best acceleration.
That's the answer to your question... But the situation is not as simple as that, we are still ignoring the thrust involved, and for aircraft that can produce large amounts of thrust, we can consider what will happen if we call the thrust t and include it. Our expression for the force now becomes:
f = mg + t - d
If we now substitute that into expression (1) we get:
a = g + t/m - d/m
What happens now depends on how big the thrust and drag are. For example, consider an aircraft that begins to dive from a low speed, at maximum thrust. Now because the speed is low, the drag will be relatively low, so just to clarify this point, lets ignore the drag term just as we previously ignored the thrust. Now we are left with:
a = g + t/m
That tells us that the thrust will increase the acceleration (once again, that makes sense) but that the larger mass makes that increase smaller.
So, if our two aircraft have a high thrust to weight ratio and start to dive from low speed, the lighter one will actually accelerate more quickly!! That is contrary to the previous situation. However, because the thrust will get smaller (for a prop aircraft) as the aircraft gets faster, and the drag will increase as the velocity squared, the drag will very quickly become the dominant factor, and so the heavier aircraft will generally very quickly, if not always, exceed the acceleration of the lighter one. Indeed, you often see pilots referring to the better dive acceleration of heavier aircraft, which supports the theory
But it does open the possibility for exceptions to the rule, a draggy aircraft with low thrust will accelerate poorly no matter how heavy it is
Now, the argument doesn't stop there, because even though one might suspect that acceleration is the most important factor in combat, the actual top speed, or terminal velocity that you can reach in a dive, might also be important?
So, if we extend the argument to include terminal velocity, it turns out that if you
are happy to ignore thrust, you can compare the ratio of the terminal velocities of two aircraft as the square root of the ratio of their weights.
So if one of the two aircraft in my previous example was four times heavier than the other, its terminal velocity would be twice as fast.
So, not only does the heavier aircraft accelerate better, it reaches a higher terminal velocity!!
That's more important than you might at first think, because when most folk consider terminal velocity, they imagine a vertical dive... I suspect that few pilots that enter a vertical power dive ever get anywhere close to their terminal velocity, at least not many that live to talk about it
The point is that there is a terminal velocity for every dive angle, dive at 30 degrees in a heavier aircraft and you will still accelerate better, and reach a higher terminal velocity for that angle of dive.
That is why the heavy US aircraft were always able to extend away from the lighter Japanese aircraft. Even if two aircraft can match speed in level flight, the heavier one could still extend away with better acceleration and higher terminal velocity, for as long as the dive could be maintained.
Hope that helps.
Leon "Badboy" Smith
