-Isel-
But, Shinya could be small due to bone structure. It's quite possible. Bone structure affects a lot... for example, that's one reason why I weigh 30 pounds more than my friend who's the exact same size I am. That, and muscle... but my dense bones affect it. Marrow content.
I have a lot of "Marrow of my Bones".
...meaning you would be quite the angstmuffin, according to ancient Roman tradition.
*rubs hands* This is where Latin comes in handy...but I'll save that for another thread.
Shinya is muscular (at least, around his arms), so we have to factor that in, too, but...yeah, he's got tiny bone structure. Here's some official measurements....

Translated to Americanized units--
... just under 5'7
... 115 lbs
... 30'' chest
...HOLY ******** A
25'' waist... 32'' hips
...a foot-and-an-inch-long circumference neck
... 9'' biceps (at time of photo. He's definitely gotten more muscular since then. )
... 5'' wrists
... 17'' thigh
... 7'' ankles
So he clearly has really tiny bone structure and probably not
unhealthily skinny. I do have a picture from the Macabre era where he's got muffin top, and he does a lot for his health. If I remember correctly, he mentioned something about resisting junk foods in an interview or two and eating a lot of fiber.