That's only true if both bullets are loaded to the same final velocity because the lighter one would have less powder hence less recoil (muzzle is rising slower). Comparing magnum to .38, magnums will print a shade higher even if the bullet weight is lighter because the significant increase in recoil rotates the muzzle up faster. Recoil wins on that one.

Saw the same thing testing POIs on standard 158 grain .38 ammo compared to the 128 grain +p defense loads. Both were hitting same POI because the lighter ones were significantly hotter loads.

But for the record, magnums shouldn't group eight inches off at 25 feet range. maybe a couple of inches max at 15 yards.

Lower left group = yanking the trigger. Take it from the man who has done it a few thousand times.....