growing potatoes in a tub

We are growing first and second earlies and main crop both in bags and in the ground. So far we have emptied the first early bag and are disappointed with the crop compared with that from the ground (we emptied the bag when we were half way along the row in the ground).
I assume that if we had left them in the bag for longer then they would have bulked up better.
The trouble with the bag system is that you cannot scrape some soil away to see how the tatties are doing and the bag has to be emptied in one go.
We will leave the second earlies and the main crop bags for longer.