Parallax adjustment is dictated by design. If there is not a focus lens in front of the erector(adjusted by the side knob typically), the parallax is fixed by setting the objective lens or by using an Adjustable Objective(AO). Magnification nor objective size have anything to do with it. Parallax corrects a phenomenon that occurs and compounds over distance. It corrects the image and brings the target onto the same plane as the reticle. The common thought process is that more magnification means that targets will be viewed at greater distance, hence parallax correction is necessary.
The NXS 2.5-10x32 had a fixed parallax of 125 yards. We heard for a few years that we needed a larger objective and side parallax; move forward and we have the 2.5-10x42.
I used the 2.5-10x32 for a few years on my lightweight hunter and could hit 10" plates at 750 yards. I had to be very conscious of my head position and it had to be very consistent to hit it. If I moved my head the slightest bit off center, I would miss.
I now have the 2.5-10x42 on that rifle and the ability to eliminate parallax at 750 cut my groups in half. I can now stack them in the center with much more forgiveness to head position. Granted, now I have to worry about checking focus at close and intermediate ranges as if I require a quick shot up close, there is a chance that it could be out of focus due to me not checking, so it could be a hindrance for some in that respect as it is one more thing to worry about.
Parallax error is relative to target size at distance. For the 2.5-10x32, I typically could not induce enough error to miss a 10" plate out to 600 yards or the scope would black out. If I was trying to hit a 5" target at 600 yards, I could induce enough error by my head position to miss. The ability to eliminate parallax is also a benefit for using reticle holdovers as the parallax error could be increased in a lower portion of the reticle.
I hope this helps you understand the intent a little better.
