Looks okay to me (but, again, the fluff)
In my experience with youngins, they tend to fluctuate a bit in their weight as they are growing, and can't really expect them to look as filled out as an adult without them being overweight.
Breed is also a factor, as some are naturally slimmer than others in their optimal weight (take a thoroughbred vs. a quarter horse). My horse stayed slim most of her life, when she was about 4-5 she thickened up, filled out more in her butt and chest- and now at 6 with her recent growth spurt has thinned down again. She is an arabx, and looked more Arab in her younger years, then more quarter horse when she started filling out and maturing more. She has never been considered underweight.
I think the scale is okay as a general reference point, but by no means matches every horse. The paint out here grew ridiculously. He has only just recently, at 7 years old, started looking normal (I've known him since a yearling, and... Well, good golly the guy was fugly, he'd have 1 month out of the year he looked proportionate to himself).
I would say, if you are really worried about it- then ask your vets opinion (since they can come and actually look).