He does look like a lot of yearlings that need to have their protein upped. Either in concentrate form or in hay form. The filly actually looks pretty good!
The type of hay seems to affect weight, too, as well as the individual quality of each bale. A flake of alfalfa is completely different in weight to a "same size" flake of any type of grass hay or timothy hay. Then, too, different balers (the baling machine) pack/size flakes different - in different parts of the country. That is why you've been recommended to weigh your hay. Each type of hay is also different in nutrient value/per pound.
What I finally found that worked on one of our guys, and in short order, was a tube of Probiotics (yucky paste, dosed for him over several days) and then being put on "Body Builder". The next thing I did was to put him in a paddock w/ his two year mates (he was born end of Aug, they born mid Apr) but also w/ 2 slightly older geldings (4yrs) who both chase them and play with them. The extra exercise means he's hungry at feeding time (he would stand at his bucket and not eat - every couple of days - SO FRUSTRATING). Then I introduced alfalfa pellets - going completely free choice for all of them (5 ponies ranging in age from 1 - 4 yrs of age and from 30 - 40" in height at the withers) - OMG that turned into a lot of alfalfa pellets VERY QUICKLY(when I actually measured it weight wise - they were cleaning up 37-40 lbs daily for about 1 month!). After the 1 yearling and 1 4 yr old gelding started getting really bulked up & FAT and the others all started looking good to great, I started cutting them all back so that instead of having alfalfa pellets in front of them 24/7 they got to the point where they literally got it added to their buckets individually after they cleaned up their feed and the two "chubs" got cut waaayyyy back.
This colt didn't go back to not eating/finishing his feed until we moved them from their home, to temporary quarters (we moved out of our leased farm on Nov 20th, ponies were moved the weekend of the 15th of Nov. The closing on our new farm keeps getting pushed back - started at Dec 4th, now at Dec 19th. Will have to put up fence before we can move the ponies in - will be a change from county water to well water along w/ different pasture. Feed will be the same and hay will be the same for now as the current supply at their temporary quarters). After 3 weeks, he's still not where he should be as far as finishing his feed and I will be getting some tummy soother for him this next weekend (pepto bismol works in a pinch, & should have gotten the other sooner but...) as he also got nasty diarrhea after this move along w/ not eating... He has lost some weight again and it shows on his top line and in his belly - same as your colt. He is on pasture (scrub/bunch grass and sand) and free choice coastal hay round bales (different supplier). Both the move and the change in hay upset his system. He is the 2nd pony I've had in 3 years that will refuse to eat at all if you take him away from his buddies - starting when he was weaned from his dam even though he was eating just fine at her side. He turned a true year old on August 31st while his "year mates" are going to be 2 in just 4 months. He has never gotten any separate exercise other than being free w/ his dam and his year and pasture mates. Once I start this guy on a tummy soother, I will keep him on it at the new place for a while and I will also have to get more Body Builder as I used up what I had (it was only a partial, older bottle).
We had one filly that was in a show barn in 2012 as a yearling and the trainer handling her put her on Ani-Meds - ulcer product (have to research the name - can't remember what it's called). I can't get it here in NC unless I have a vet scrip and then have to order it. Unlike in the mid-west, it isn't carried by tack/feed stores here - and the two that I frequent the most said that they can't just order it in unless they have a scrip on file. Made a huge difference in her - not only in body condition but also her personality... It was not a drug, but is (or was?) a feed supplement. Maybe now it's considered a drug? and that's why it needs a scrip here? I sold that filly with her dam last Christmas.
Hmmm, I just went thru a lot of pictures - it seems that here in NC, I've had problems of one sort or another with all of our later born foals (aug/sept) when they are in their "yearling" year. ALL have developed some type of belly as well as going off feed at one point or another. In some cases, simply switching a wormer or doing a Panacur PowerPac worked. Others required a huge boost in protein and/or some type of belly soother. ONE had a tumor in his abdomen and at 19 months of age was euthanized (not from our breeding program and dam not re-bred before sold). This was a total of 8 ponies/minis over a 17 year period and living on 2 different properties. 4 of those were purebred Shetlands (3 are "B" sized minis) in 2011, 2012 and 2013... Very strange and another reason for me to love having babies between January and April!! (though that probably had nothing to do with it overall...)