That use of the measurement line... The two Muskox are so close that perspective doesn't really play a noticeable part. So, sizing visually makes more sense. As does applying logic.
But I'd add
measurement to
visual whenever possible. In this case, I used the distance between the eyes as a comparative guide to size.
Many years ago I saw a delightful drawing of Mastiffs arranged on a set of stone steps. But the drawing was ruined by the wild inaccuracy of the dogs' sizes that, compared to the steps, were far too small.
Taking those steps as an example, they have one known constant that we can use - the height of the riser. Stairs in your house most probably have 9" (23cm) wide treads, and 9" (23cm) risers. However, decorative stone steps might have wide treads, but I can almost guarantee the risers will still be 9" high. Bear with me for a moment...
The treads can be any depth. It doesn't matter if you have to step up and take a couple of steps before stepping up again. But no step is going to have, say, a 12" (30cm) riser, or greater. Because climbing up steps of the height is awkward and tiresome.
So that artist could have compared the height of his steps against the published breed standard height of that type of dog.
As long there is something man-made within your scene, you can almost always take measurements from it to resize other imported elements.
Now, while I'm on a roll... notice also that
your drawing does not contain anything recognisable at all. Just the two Muskox - or Muskoxen? What is the plural?

And I think that's one reason why the use of perspective was rendered almost unusable. Our only clue to scale is the two Muskox. So, we're forced to compare one to the other to ascertain their relative sizes. And, because one is noticeably smaller, that's probably why it appears to be younger.
Artistically, I do prefer the one I drew with the left one appearing much smaller
Good. That's all that matters. And, as you explained that the male is physically 20% larger than the female, it all makes perfect sense.