It has to do with reticle subtension at the field of view of the various power settings. As you can see the 2.5-10 has a much larger field of view than the 4-16.
The 2.5-10 covers the folowing:
47-10.9 feet @100 yards
4-16x the following:
27.4-7.4 feet @ 100 yards
Generally the higher the power the less the reticle can cover at 100 yards given the field of view of the optic.
Each reticle has to be designed to work well at the max and the minimum power range for each scope. They can simply fit "more" reticle in a 2.5x-10 than they can in a 4-16x.
The thickness of the lines also needs to be adjusted to the power zoom range. That is the biggest negative issue with 1st focal plane reticles. It is virtually impossible to create a 1st focal reticle that is "perfect" at all zoom ranges. It is going to be too thick at higher levels or too thin at lower levels depending on how it is designed.
If you want to see how nasty a 2nd focal plane Mil Dot reticle is that was poorly designed for the power range of the scope, look at the Sightron 8-32x. The dots are HUGE to the point of being pretty much obnoxious and un-useable.
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