Looks nice, autos spend most of their early life getting a good root foundation, plant them in a nice big pot and leave them in it. They do not like re-potting.
Second big thing is watering, if you gave it a good watering when you planted it you may not need to water again for over three weeks. A good way to check is dip your finger into the soil around the edge of the pot down to about an inch, if the soil feels moist it does not need water! Less water the better, the dryer the soil the faster the roots will grow. more roots = bigger plant = bigger yield.
Don't feed the plant any nutrients until flowering, they need very little until this time and should have enough even in bad soil.
My experience with auto's is leave it the f*ck alone until it gets to a decent size! Just leave it in that pot and give it light for about a month, you should have a nice little plant going.
If the other plant is not of the same genetics there is no point in comparing them, different auto strains can grow at extremely different rates.
Fact that its small might be a good thing if its getting a good root ball going. If there's anything you might want to do its put it closer to the light but not closer than 4 inchs.
Happy Growing!!