So I decided to look up some basic Korra design's again in order to find out what exactly it is that makes his Korra look always more boyish than he may intend.
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And I think the problem is mostly related to the relative length of how he draws the hair, the linework in the face, the nose, the chin and the size of the eyes.
First, the hair is supposed to be cut in a certain ankle so that it actually is shorter around the backside of the head and gets longer when it comes to the front. This illusion is not given in most of Dobson's drawings. Additionally, because the top of Korra's head is flatter than in the original design, it creates a more boyish illusion.
Which is also created by additional facelines, making her face look harder than it ever is intended to be. Korra's nose also stands u a little bit at the end, always making it look sharper and more prominent than it should be. The form of the face when it goes over from the cheeks into the chin are a bit too sharp than intented and the eyes come of as bigger than they should and lack a certain amount of feminine streaks (Eyelashes e.g., even if they were not as prominent on Korra ever).
It also doesn't help that Korra's waist e.g. is bigger than in the original design, therefore hurting the feminine design.