Hi everyone, thanks for the replies.
Yes, Lilly is definitely lacking muscle. Her owner was pregnant and didn't do any riding during that period, so she hasn't gotten much exercise in the past 9 months (I definitely plan on changing that!). That is part of the problem. However, the pictures ARE bad quality/at a bad angle, I am almost positive she is underweight. She appears that way to me, and nearly everyone who's seen her has mentioned something about it. Although these pictures don't show it, she's very hollow and sunken in right at the flank/end of her ribcage where she should be looking full and round... also, from these angles it doesn't show, but you CAN see a light outline of her ribs in person -- my thoughts are, she's definitely not "lean with muscle", therefore she must be underweight..? I think I am going to ditch the senior feed and switch to alfalfa cubes thanks to cloudkisser's advice :) I also think I might add flax seed because I hear it does wonders for horses, from the inside out.
I do secretly hope you guys are right though... I don't want my baby being underweight :( I wanted to take lessons, but right now I'm foregoing them because I'd rather put my money towards getting her back on track nutrition-wise.