^Yeah, very nice looking & rather 'thick set' horse there! You may want to inform your BO that there are healthy, low carb alternatives if she wants to give treats. And there are many alternatives to grain for horses that need to gain weight too. I agree the horse doesn't look like she can stand to get much heavier. I'd be concerned about IR & associated issues if she is like that most/all the time.
As with us, animals can afford to get fat, particularly if they exercise a lot, without it affecting their health, but it's staying that way long term, without regular 'hard seasons' to use up a lot of the fat stores that is most problematic.