she isnít obese but her belly is looking very round and her crest is big.
Belly being round doesn't necessarily say 'obese' but if she has a crest, particularly if it's big, that definitely does. If she has a fat crest, she also may have 'fat pads' on her shoulders, sides & rump. Has she? Is her crest & these pads soft & spongy, or firm/hard? Pictures?
If she's cooped up in a stable every day, only allowed out at night, when they move less anyway, she is getting very little exercise, especially if she's kept solitary - you don't mention if she's out with mates at night. I'd want her fully turned out if possible, in a herd situation, which aside from being good for her mentally, will motivate a lot more movement. As well as 'working' her more.
Re feeding, just replied with relevant info on https://www.horseforum.com/horse-hea...-807463/page2/
We have a covered horse walker, an outdoor school, a lunge pen, a gallop track, and plenty of on and off-road hacking!
While a horse walker is great for exercise, I'd rather take my horse for walks - not boring. Along with more exercise for me too! Depends what you do in the 'school'(arena?) as to whether that may be boring, or how much exercise she gets there. I would do little lunging/circling with an unfit horse especially, as it's hard on their joints. And it is also boring to just do circles for exercise. If you can brisque walk or trot on the 'gallop track', go for it, but I possibly wouldn't be doing much cantering or any galloping if she's really fat. If you can ride her, go hacking, particularly long trotting & hill work, great. Or do this, or ground drive in hand or pony her off another horse.