First off, 3200 fps with a 200 gr Accubond is certainly a practical load in a 300 RUM with at least a 26" barrel length. I have build dozens of these rifles using Lilja barrels which are not known to be fast barrels and most of them easily reach this level of velocity with that bullet weight.
Now to the WHY its possible.
Well, there are two schools of thought. One is the larger surface area on the bottom of the bullet allows more pressure to be applied to the base of the bullet which results in more velocity. There may be something to that but I generally fall into the second school of throught.
In my opinion, its all based on sectional density and bullet baring surface to the bore. If you take the same case capacity and two different caliber rifles, in this case 7mm and 30 cal and shoot bullets with the same sectional density, you will come up with nearly identical velocity results in most cases all else being equal. If you look at bullets in 30 cal and 7mm with the same sectional density, the 160 gr 7mm and 200 gr 30 caliber are very similiar. As such, you will get similiar results.
In my testing of the 7mm RUM with the 160 gr Accubond, in 26" barrel lengths it can easily be pushed to 3200 fps and often slightly faster. In fact I would say on average, from the rifles I have built and tested, 3250 fps is about right for the 7mm RUM with the 160 gr Accubond so it will get you a bit more velocity then the 300 RUM with the 200 gr Accubond but the 30 cal has a slight edge in Ballistic coefficent so its pretty much a wash in the end.
Barrel life is in the favor of the 30 cal but not dramatically. The 30 cal can be a bit less effected by enviornmental conditions simply because it has a higher expansion ratio compared to the 7mm.
Both can be made to work extremely well, both are extremely accurate in a good rifle. For heavier game at long range I would opt for the 30 cal personally but either will do very well.
