For a given bullet weight and pressure, the larger the diameter the faster the powder can accelerate the bullet.
So, a rifle can accelerate a 300 grain .45 bullet faster, at the same pressure, than it can a 300 grain .43 (.444 Marlin) bullet.
This comes under what
they call internal ballistics.
When it comes to external ballistics, the 300 grain .43 bullet has a fractionally greater length and therefore greater sectional density, and it could arguably retain its velocity a little better than the .45 bullet of the same weight and general shape.
Terminally, the 300 grain .43 bullet, by virtue of its slightly greater sectional density, would penetrate a little deeper; whereas the 300 grain .45 bullet would create a slightly larger diameter wound channel.
Theoretically, given a modern rifle, one could develop slightly hotter loads for the .45-70 than for the .444 before he ran into pressure problems.
I say theoretically because so many other factors can enter into the equation.
If reloading and pressures don't matter, one should pick the caliber based on what matters most to him, such as the types of rifles chambered in that caliber.
One might want to remember or think about the fact that the larger the diameter or surface area of the base of the bullet, the less pressure it takes to push it out the bore.
This remains true of all straight-walled cases.
When considering bottle-necked cases, the diameter of the case becomes a factor: expanding gases in extremely large diameter cases do not push small bullets as efficiently as large bullets.
For example, the .30-378 and the .338-378 will produce very similar velocities for a given pressure, but the .338-378 will do it with a heavier bullet.
I hope I have all the above right.
I remember it from a book by P. O. Ackley I read about 12 years ago and so I might have some of it wrong.