That's an interesting question. Let me see if I can wade through a few issues.
By "bigger" I take it you mean higher magnification. The March-X scope to which you refer is the 8-80X56 model. I have the 5-50X56 version of it. The two scopes are very similar in dimension and weight so making a 10-100X scope would not be a bigger or heavier scope on the face of it, but there are other issues to consider, which we'll get to in a bit.
To my mind, the most important feature that allows these scopes to have such high magnification is the zoom ratio of 10X. The most common ratio is 3X with 4X being almost as popular these days. There are a few that are running 5X and even 6X, but the March is the only one with a 10X zoom, to my knowledge. I'm sure others will chime in to add to my deficient knowledge.
The way a riflescope works is that the objective lens forms an image in the first focal plane, that has a diameter equivalent to the inner diameter of the main tube. Think of it as a camera lens focusing the picture of the objective on the sensor or film. The focal length of this portion of the riflescope is quite low in comparison to the actual magnification of the riflescope. It's the distance between the objective lens and the first focal plan, the front of the erector cell. If you compare that to a spotting scope, you can see that the spotting scope is longer and so has a longer focal length built into the body, but its first focal plane is much nearer to the eyepiece where it flips the image with its prisms.
In the riflescope, the inner tube with its erector cell and zoom lenses in front of the eyepiece are really used to inspect a smaller portion of the first focal plane image, that's where all the magnification magic occurs. If you compare the 8-80X56 to the 5-50X56 you will notice that the only difference in dimension is the distance between the adjustment knobs and the zoom ring. The 8-80 is 9mm longer in that sport, from 51mm to 60mm. Everything else is the same. What this indicates is that March increased the base magnification from 5X to 8X by increasing the focal length of the lenses for the eyepiece.
I have not look through a March-X 8-80X56 so I do not know how dark it is at 80X but I can detect a change in my 5-50X56 which I usually run at 40X, when I go to 50X. The image is fine, but darker.
The size of the exit pupil is predicated on the size of the objective lens divided by the magnification. At 40X, my 56mm objective form an exit pupil of 1.4mm. It's nice and easy for me to get behind on the rifle and it still projects a good amount of light that I enjoy the picture. At 50X, the exit pupil is 1.12mmm.
At 80x, the exit pupil is quite small 0.7mm. Getting behind the rifle with that small an exit pupil is exacting.
At 100x, the exit pupil would be 0.56mm; the criticality of it would be frustrating for most shooters. So we could always increase the size of the objective lens and from 56mm we could go to something like 72mm, like some older European scopes used for hunting at night. That would make the exit pupil size as 0.72mm, the same size as the current 80X setting.
Of course, when we get into such magnifications, the atmospherics come into play, big time thus negating the benefits of such high magnification a good portion of the time. Plus it accentuates any quirks or tremors from the shooter.
However, all that aside. I think we are approaching a juncture here with riflescope where the paradigm is going to shift. The front part of the scope, from the objective to the sensor or the erector cell will remain and may evolve to be closer to camera lenses. However, from the sensor back to the eyepiece, we should see a change to digital image processing where that part of the riflescope will be concerned with zooming in a cleaning up the portion of the sensor that we are using to aim the rifle.
There are some interesting times ahead, but the current riflescope is not going away even if it doesn’t gain any greater magnification.