Always make sure you check their feed at the end of the day
If you work them after you've fed initially you need to feed again as they will need more oats

From 3 years of age, a horse needs to weigh its height x 70, with it being 110 for drafts. Are you checking at the end of the day to make sure that they've had the correct amount of oats?

If you don't want to take the time invested in doing it this way, then just use a black orchid and it will reset everything for you.

A Black Orchid will only restore the energy, health and morale levels. It won't fix the weight issue. That can only be done by adjusting the feed amounts

if you do anything AFTER feeding you will need to feed more oats. so you either have to feed twice or when you feed feed more.
I feed once (early) and this is schedule for feed/work/care i follow and don't have a skinny horse issue.
1. groom. do something that takes energy to around 70% energy. lesson, 1.5 hours ride, some training (how much depends on which one you do), or 1 or 2 comps (depends on how much energy is used) works really well for this. I take it down this low cause you get the most energy from feeding when energy is highest. but you also don't want it to high cause you can't go over 100%. so with the amount i feed in step 2 i found 70%ish to be a good amount to feed at.
2. feed required fodder and 10 oats. I feed 10 oats even though it will only say 1 or 2 so i don't have to feed again. Also gives the max energy boost i talk about in step 1. if you wait to feed the rest til the end you won't get the same energy boost. I have found 10 to be pretty average. sometimes need more sometimes less but it evens out.
3. work the horse like i normally would til energy is either as close to 0% or it's 2100.
4. water, carrot, stroke. stroke energy gain works the opposite of feeding so you want to do stroke when the energy is as low possible. W/C/S gives right around 20% energy and takes 1 hour. that's why i say work til energy is 0% or it's 2100. these 3 things will take you to 20% or more and to 2200.
5. if it's before 2200 and there's more than 20% energy if there's something else i can fit in i will. like if it's before 2145 and has 26% energy and still needs dressage training i'll throw in a half hour of dressage.
I only use mash if it will get me more. The way i do my horses it won't usually do much so i don't waste the mashes.