Dgang,
You seem to be laboring under the false impression that only bottleneck cases are loaded to high pressures. In the old days, if pressure was to be increased by adding powder,
they just made the case longer. Today, the 460 S&W Magnum, a straight wall revolver cartridge, has a SAAMI pressure rating of 65,000 psi, the same as the 270 Winchester. People have loaded the nearly straight wall 45-70 to over 50,000 psi in the Ruger single shot rifles for decades. So no, bottleneck don't make pressure higher.
The idea that a small hole in a chamber volume raises pressure in and of itself is like saying the pressure in your air compressor tank jumps when you fill a tire, because the hole you are bleeding the gas through is small. No. That doesn't happen. The pressure just bleeds down gradually. That's all. Use a bigger hose and valve to let the air out and it bleeds down faster, but that's the only difference.
Imagine a case and bullet with a super-fast powder like blank powder in it. You put in just enough to meet 60,000 psi, and because it is so fast that pressure is reached before the bullet moves even a hundredth of an inch. Now you have a pressure vessel filled with pressure and it won't start going down until the bullet starts moving forward. Note that I didn't say what size the bullet was. This could be in a bottleneck case or in a straight wall case. Either way, the pressure is the same.
The above dynamic only changes when we move to slower burning powder. Slower burning powder lets the bullet start to move before the peak pressure is reached. As a result, the pressure is reached in a larger volume, the case volume plus the amount of space added by the bullet moving down the bore. That means we can put enough powder in the case to reach the peak pressure at that bigger volume. It's too much for a bullet that doesn't move or that doesn't move quickly enough, which is why a heavier bullet raises the pressure a charge produces. Once you start counting on that extra space to let you make more gas (which does increase the speed of the bullet over what the super fast powder can give you), you have to be sure it is there. Well a smaller bullet of the same sectional density would move just as fast under the same pressure, but because its diameter is smaller it has to move faster to provide the same extra volume. So you either have to use a bullet with less sectional density or you have to use an even slower powder that gives it time to make enough volume to keep the extra gas you are making from driving your pressure up too high. This is why a smaller bullet in the same parent case (say, a 22-06 verses a 30-06) requires a slower powder and/or a lower SD bullet. Usually it is some combination of the two.
In the case of the straight wall cases, then, reaching a high pressure, if the gun can take it, is just a matter of choosing the right powder burn rate and the right bullet sectional density.
Regarding smokeless powders in general, due to differences in their formulation, they contain anywhere from just under 3600 to just over 5000 joules per gram of potential energy. So yes, sometimes the trend to use slower powder in heavier charge weights than fast powders doesn't play out as expected, and that is generally why.
In this, I haven't gone into the effects of progressive burning verses slow-but-digressive burning powder on pressure and velocity, so there is more for you to dig into.