For gamma plugs, the threads carry the pressure stress and the fillet is only a seal weld. The 3/8" fillet size is usually the maximum size to avoid PWHT. The force (pounds) acting on the the threads is the pressure (psi) times the gamma plug hole area (square inches).
For an o-let or socket weld, the pressure stress is acting on a smaller diameter pipe than the main run. The stress level in a straight run of pipe is calculated in simple terms by an equation that looks like this:
Stress (psi) = [Pressure (psi) x radius (inches)] divided by wall thickness (inches)
There may be some additional terms in the equation for safety factors, corrosion allowance and thermal expansion coefficients, but the above is the basic equation for pressure stress in a round cylinder (pipe or vessel). As the diameter (and radius) decreases, the stress level decreases and the required wall thickness/weld size can be thinner for the same pressure.
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