The plant in natural light (full sun) is probably getting more daily light than the plant that you are providing light for. That could be why it has grown to the same size in a shorter amount of time. It’s getting more fuel (light) to grow faster.
I recommend getting a light meter or installing a light meter app on your phone (I use a $5 app) that measures PPFD for the light your using (full spectrum LED, Sun, Blurple, etc)
By using distance, an adjustment, or both you can get the light intensity that creates an environment the plant can excel in. Could be you need a more powerful light to create this optimized environment.
I use an HLG light. This is their recommendation:
Seedlings, Clones Under 100 PPFD
Veg, Greens 150 - 400 PPFD
Extended Veg with healthy plants 350 to 500 PPFD
Flowering 400 to 800 PPFD
Flowering with Supplement CO2 600 to 1100 PPFD
For autos flipping to 12/12 for flowering is not required, but for photos when you flip to 12/12 you should increase the light intensity because they are getting 50% less light per day than they are at 18/6.