Her conformation, persay, is not changing...as she puts on weight, she will look alot different because areas that aren't filled with muscle right now, will be then.
Her "3 years ago" pic shows what I would want her to look like...not overly heavy, but nicely filled out, and just needed a bit of work to build the muscle up a bit.
If you have owned her all this time, might I ask why she's gotten so thin at times? Has she had consistent wormings, teeth floated (when needed), etc. I think if she were mine, and she was that 'up and down' I would be searching for some answers.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."