what a unique gardening container! and that little frog is so stinkin' cute!!!
i would wonder the same thing as far as amount of sunlight in regards to growth of your plants. and even though 8 inches of soil depth might seem like enough, roots really do like to dig down deep. I also think that perhaps in containers it's easy to keep the soil moist, but I've found that letting the soil get a bit dry (not bone dry) between watering, forces the plants to dig their roots down deeper to look for water which in turn helps build a good strong root base - which makes for healthier and better producing plants. That's my experience at least. if the soil is moist all the time, the roots don't have to work very hard to get a drink and don't seem to be as strong.