How can I get a bigger harvest of vegetables

You may be using the wrong fertilizer. Or using too much. Your best bet is to make sure you start with good soil. Amend your box in the fall after all the plants are spent and add coffee grounds and vegetable scraps throughout the winter. I have found that grinding them up in a blender will have the completely decomposed by spring. For now, try grinding up some banana peels with calcium (can use spoiled milk or calcium tablets) to give your box a boost. Add plenty of water or your blender will hold it against you for a very long time.

Is your planter box big enough to develop good root systems for mature sized plants without over crowding? Do you foliar feed or feed the roots when you fertilized? Foliar feeding feeds the leaves, not the roots, so you get great greenery but little or no fruit on your plants. You have to fertilize boxes or pots because they can only take in what nutrition they can from the soil and it depletes quickly in pots. Use a fertilizer for vegetables and tomatoes. If you use the water soluble type I would use the recipe for feeding each time you water to give them a little more boost. See if that helps, I hope it does! Good luck, you still have time to have a decent harvest!

Your yield results could be the results of your soil, the variety of plants you're choosing or the type/size of your containers, Marianne. See if you don't find this helpful: http://agrilifeextension.tamu.edu/wp-content/uploads/2016/05/EHT-062-vegetable-gardening-in-containers.pdf

Not enough space for the plants to grow, they need more space to produce.

What kind of fertilizer are you using? Are you are using Miracle Grow? You should try using a 'bloom booster' fertilizer. Something with a higher middle and end number on the ingredient list. Miracle Grow makes a bloom booster and tomato fertilizer. Try one of them.