Horse looks a little small!

Not necessarily. Lighter and shorter horses were often preferred by light cavalry. Heavy cavalry had huge heavy horses, light cavalry had smaller horses, but we get one size fits all in model-building. That's a problem with some companies, we get these huge destriers when the real horses were often so short the rider's feet would almost be dragging on the ground. Or we get beautiful Arabians pulling a wagon. Like people,horses come in all shapes and sizes. I'm still working on doing late Napoleonic horses which were substandard and tiny.

Then it's not really a horse ("A horse is a horse, of course, of course" ), but a pony.

Depends on the context and the breed. All ponies are small horses but not all small horses are ponies.