Yeah, as we don't know what your 'jugs' are like
, and also different feeds are more or less dense, it's best to weigh it rather than go on volume.
Firstly, just because you can see a horse's ribs doesn't mean they're necessarily underweight. It may be that your farrier is right, that she's in need of more, or he's just one of those who can't stand seeing ribs in any case. Check out 'body condition scoring' for looking at the whole horse to tell if she's good, bad or indifferent. It's also OK for horses to lose a bit of condition over winter, particularly if they were overweight in the first place. It's not good for them to be 'mud fat' all year & this is what can cause insulin resistance.
As Kymba said, it's important to understand how horses' digestion works, what is natural for them & what is not healthy. One good site on this is safergrass.org and there is also a fair bit in the articles section on diet as it relates to health on hoofrehab.com There are also programs such as FeedXL.com which I personally subscribe to, which will help you work out what feed is really what, work out a good diet specifically for your horse & situation and answer all questions.
One thing about horse's digestive systems, they don't deal really well with rich, starchy, sugary feeds. What you're feeding - Easi Result being high grain including corn & molasses, pollard(what type?) & wheaten chaff(I presume that's what you meant, not wheat AND chaff?) is quite rich & also unless you're feeding a suitable supplement to 'fill the gaps', it is likely to be quite imbalanced. Especially if you're going to feed rich feeds, it's very important to feed little & often, so I'd want to divide that into at least 3 feeds daily. But frankly, I'd be changing her diet to ingredients which are low 'GI' and adding a good complete supplement anyway. If you do need her to gain weight, I'd start out adding grass hay & a bit of alfalfa.
Other considerations you may not have thought of are that she needs her teeth done, that she needs worming, that she has ulcers or such or she's losing weight because of insulin resistance or such.