Yeah, body condition can be hard to judge from photos sometimes, depending on the breed. Some breeds are just gloriously beefy, some Arabians look like they could blow away. Generally, there shouldn't be so much subcutaneous fat that it starts to obscure muscle contours. When a horse starts to get out of shape because they're not being worked I find you often see it in the neck losing visible muscle first. The boys can get some fat at the crest of the neck but the rest of it will start looking skinny, and their posture always suffers and they start falling out of collection. On the other hand, a horse that's consistently active and well-muscled can often carry a bit more weight and look even better for it. They should never look flabby, but there's nothing better than a sturdy pony fat and glossy on good pasture and ready to go. Muscle should always be visible, seasonal coat considered.
Just stay on top of any fluctuations, work consistently for targeted muscle development to support any weak spots, and talk to your vet about diet. When judging, if something is very wrong, you'll see it. Look at enough fit animals and you'll know when something's up.
