Nothing is wrong. The visual model part was made for FS98 era models and was never updated for things like FSDS2 and gMax models.
This makes thing a bit trial and error. If you are unsure of the aircraft's size/scale frome doing say a port from FS2002/FS2004 you can use the [LIGHTS] in the panel to map out the aircraft than transfer the numbers to DPED to layout hitboxes, guns .etc
/ Panel light set to use the muzzle flash
[LIGHTS]
panel=0.0, 0.33, 1.33, fx_mzlflsh_s
/ Lateral (m), vertical (m), longitudinal (m)