when i start my seeds or clones i put them in a small pot about the size of a cup, then when they are 3 inches high i transplant into 3 gallon pots for the rest of the grow, the reason we dont start plants off in these 3gallon pots is because its a whole lot harder to keep track on watering, the chance of having something rotting in the bottom of the pot is high, as the plant will only be young so the roots will not be that deep, so the rest of the pot would be holding all the moisture, this could also rot the roots of the plants, its harder to manage the moisture in the soil if you have a new plant in a big pot, so i always start them in small pots then transplant,
i never have transplant shock either, the best way to transplant is to let the soil dry out, dont water it for a few days then transplant when its dry, then all you need to do is tip the pot upside down into the palm of your other hand with the stem if the plant between 2 fingers, then tap the bottom of the pot and give the sides a gentle squeeze, the plant and soil will then fall out in one lump, this stops any shock as your not breaking the roots up, if the soil was moist when you did this then it would all fall apart in your hand and cause lots of shock to the plant, so by letting it dry out it stays in one lump and is very easy to transplant.
starting seeds in big pots makes it difficult to get the watering right, you wont know if it needs watering as the soil will be moist all the way through so this can make people over water as it can take a couple of weeks for the pot to dry out with just a seed in it, but with a small pot it dries out in a couple of days and this is what the plant and the roots want, the roots need oxygen, so having the soil to wet will stunt growth and cause root rot, you need to let the soil dry out between watering, doing this in a big pot is hard when its just a seedling,
with my autos i start them in a seedling tray then transplant them into 1,5gallon square pots, these dont hold that much soil so dont suffer with moisture with a young plant