I was taught to have one plant per container. So the plan goes like this, plant a few seeds based on how old the seeds are. Let them sprout. Let them get to first true leaves. Then the hard part, thin out the weaker plants. If you don't thin them out then the roots will compete with each other and you risk losing out altogether on fruits. I suspect the plants in the back are just doing better with being crowded. Those back plant leaves are larger it seems. So they're cruising, or they're just a different variety. The front ones are doing worse because roots are too packed together. Sometimes you must snip them with scissors to thin them out so you don't mess with the roots of the one's you want to keep. You may be in this position now. Thinning may seem cruel, experiment with it and you may come to the conclusion that it is necessary for many tasty tomatoes.