I don't believe it would produce a plume that large if it was that small. If it was coming from the shoulder vernier, then yes, that plume is about the right size for that nozzle.
If you guess the origin of where that plume is coming from, it would be right at the tip of the shoulder, right where that shoulder vernier is not the FAST pack.
if you take a look at the shoulder detail from the yf-19 model in the june 2002 model grphix, it looks like it has three seperate thrusters... I think the plum comes from the rear facing shoulder thruster and not from the fast pack.
zoomed in pic:
Interesting....