If you have used Purina Equalizer, have you ever noticed your horse gain weight on it.
I have been told it's just an overall balancer but have noticed that my mare is now turning into a balloon. Other things that have changed since she was at optimal weight have been that she is on a round bale, however I have introduced a grazing muzzle to prevent gorging.
I have used purina enrich 32 for a super easy keeper and he did fine. Make sure you are only feeding what they need. Mine is small, so he got 2 cups per day. 3 cups is a little over 1 lb, which is the recommended amount for most horses. If hats not the problem maybe the hay has more sugar than previously. You could also just use a multivitamin/mineral supplement w/ no sugar fed at just a few ounces. It depends on what the horse needs. If they are in decent work the protein of RB can be very helpful to keeping them in good shape.
I am in Canada and not sure if that makes a difference. The Purina nutritionist recommended feeding her 5 cups of Purina's cups every day based on her condition and work level. I cut that down as she started ballooning. The bag definitely says Purina equalizer. http://www.oldco-op.ca/images/Equilizer.pdf
I think if I fed 5 cups to my easy keeper I would not be suprised if he gained weight. What sort of work is she doing?
Maybe try backing down a bit, my 500 kg guy did awesome on 2 cups working 5 days/week. Each horse is different and the recommendations are starting points, if you end up close you're doing fine bc it's not an exact science with all of the variables involved.