My personal feeling about this is that it will have more to do with string tension and instrument set-up than with scale length, just based on having played multiple scale lengths from double bass to mandolin with great action. Fret size might also be a consideration.

At any rate, I've heard Fagan play several times, and watched him up close, and his considerable talent and experience has more than a little to do with his abilities.

Not knowing your abilities, consider how close to the fret you hammer-on, the degree of force you use and how long you hold the subsequent note. Also consider whether your pull-offs are actually pulling the string slightly as if to pick the string with your fretting finger, this will help get a louder note so long as the fretted (or open) note is held.

What James Fagan tune are you playing?

Peace, Mooh.