My father grew up on a farm that was mostly for subsistence. He said to pinch off the first few green tomatoes to improve yields. I occasionally do this but have not tried to measure any yield differences.
In my experience that tends to encourage more, smaller fruit; which is good cuz it reduces loss to insects etc (they ruin a whole fruit at a time). If you want a single tomato the size of a child's head, tho; let it keep the first few it sets and then pinch flowers before they open after that.
Maybe because of all the slicing and chopping we often do to tomatoes before we eat them.