On the micrometer scale of bacterial sizes, you could pack one green and one red bacterium quite closely together without observing much effect of one on another. There is transfer in all directions, and hindering on just one side is not very significant. However, this is not realistic because there should be many more bacteria. The next sketch lets you change the numbers of cells.
Note that two cells that are near each other can greatly distort the contour lines. Whereas there is some hindering with just two bacteria, there is plenty of hindering when many are closely packed. Now the less efficient guys suffer in several ways. First of all, the efficient bacteria are in the way and intercept the nutrient. This works both ways; the less efficient may also intercept some nutrients. However, the more efficient cells can compete quite well at lower nutrient concentration. This causes the second effect where they use up nutrients, lower the amount remaining, and place the less efficient cells in jeopardy sooner. With a growth rate advantage, the more efficient cells can take over the population fairly quickly. If they lower the nutrient concentration so that they can still grow while the less efficient cells have only enough for maintenance requirements and are not growing at all, the effect is enhanced.