I think she looks fine. Some horses are just naturally skinnier than others. Seeing a little bit of ribs is no problem. She is being fed the right food and the right amount and she is being exercised ans she is healthy is every other way and has a lovely coat so she is fine. I wouldnt worry about it
I agree Heather, If she is something being ridden everyday and is at a boarding stable where someone else is caring for her she could be chubbier. Obviously since we know first hand how fast our riding horses went from great to 6 months of weight re-gaining costs.... I don't like seeing her ribs and her hips stick out a little. She looks like she's on good quality with the shinny coat, but not quite enough to keep up threw the winter and with heavy riding.