She definitely NOT light. She appears to have a Winter coat so it's hard to make a close observation from just the pictures. If you can feel the outline of her ribs when you run you hands down her side with light pressure then she's about right. If you have to press a bit to feel them then she could probably drop a few lbs. The rear shot is a good indication that she's either about where she should be back there or perhaps a few lbs over. You don't want a crease (to much weight) and it's best not to have much spine.
General rule of thumb is much like with humans... a little ligher is better than heavier.
I have a picture under Val of my two horse that gives a good example of one (Kit on the right and partially blocked) at text book correct weight and the other (Val on the left) needing to lose at least 100 lbs. I'll post it here and hope that it's not too large for here. If it is so large that Kit is clipped off you can see it under the photos I have for Val.