They look thin and spindly because they're too many in the pot. A single dwarf/miniatures sunflower can be planted in a 12 to 16 in pot. Mammoth sunflowers need a container that is larger (like a half whiskey barrel). Generally speaking, large sunflowers do not do well in pots as their roots can grow up to 4 feet deep require a lot of water & are heavy feeders... I'm not saying it can't be done successfully, but don't expect them to be as large as they would be if planted in the ground.