Start in the absolute smallest container and gradually increase size each transplant. First - transplant shock is much more legend than reality. Ham-handed people that know they are klutzes be advised that by taking your time and knowing what you are doing beforehand even you can transplant successfully. Read, read, and then read some more before asking questions but we welcome those too.
However the absolute prime reason is the rapid development of a big and healthy root system. Big Steve advocates for this container stuff on here and I openly derided and mocked him for it. On here. Then I was forced, literally, to use a box of 4" planters I had when a blizzard here in AK shut down all unnecessary travel for days. Jesus H Christ. I ran out of the 1 gallon "seedling" planters I had and was overrun with seedlings. Out came the seedling soil and the 4" planters. Let me assure you that by the time all these plants were 1 month old those that started with Big Steve's baby container bit were far bigger and healthier than their 1 gallon sisters. They all, to a one, required moving to a 3-gallon much sooner than those that were placed as seedlings (all were equal now at that point) in 1 gallon pots. Everything else was the exact same. Lights, distances, water/feed. Everything but original container size, 25 plants. 13 in 4" and 12 in the 1 gallons.
Yield was much better and bud quality was better on the ones started in the small units too because it developed sooner and bigger and then used the remainder of flower time to fill in.
No secret it's roots before shoots. Use the graduated container method and get them bigger and faster!!!!!