fat---chubby---slim---skinny (undeveloped muscular build)
fat---chubby---lean---muscular (developed muscular build)
In light of what the dagreenpillow says, I don't think fat is the correct term, I believe they're actually chubby
1)muscles do not show
-it is a given that when a person is slim, their musculature is apparent even though how undeveloped it is, specially the very sexy linea alba
2)the abs, again, the linea alba
3)thickness of the thighs are thicker than their heads...means a lot of fat is accumulated there, in general they should be the same in width
-in accordance to this, the sexy muscle, sartorius, does not show, that is the marker for low fat, high muscle density (lean) in the thigh area
-specifically, the sartorius marks the change of shape in the thigh from the bulky quadriceps from the small adductors.
4)the upper arm does not show muscles means her biceps and triceps are coverred in fat or they are not developed but also putting in their relative thickness with the head, it's x>50% of the head thickness in your images when it should be x<50% if they are low in fat.
-for additional info, the sexy muscle here is the junction between pecs, latissimus dorsi and the arm...which is the armpit
and you didn't really show that in the last pic and instead made it with one small curve, thus making muscles not defined and giving an illusion of fat in the area
(The logic of the sexy muscles is the hidden principle in the zettai ryouiki (legs) and armpit fetish that has been going on in moe styles)
Now with my knowledge on physical fitness and fat distribution, the major fat centers of the female body is the triceps, the suprailiac and the thighs...funny that those 3 specific areas are the areas that you did not show musculature. D: (Note that the suprailiac is the area of the sides of the rectus abdominis (abs)...for more info those areas are covered by the obliques.
I used the 3 site skinfold measurement as basis, you can check here for the centers of males as wellhttp://www.dietandfitnesstoday.com/skinfold3Site.php
Hope this information helps in making your characters look their best.