This is an easy point to misunderstand.
The size and shape of a fusion bead can vary depending on pipe diameter, wall thickness, SDR, material, and the fusion procedure being used.
So a larger bead should not automatically be taken as a sign of better fusion.
What matters more is whether the bead is continuous and reasonably uniform, and whether there are any visible abnormalities around the joint.
It is also important to remember that bead appearance is only one part of the inspection process. Joint quality still depends on proper pipe preparation, heater temperature, heating time, changeover time, fusion pressure, and cooling time.
Where ISO, DVS, or another specified fusion procedure applies, those process parameters should still be followed accordingly.