If you think about it logically, it should be easy to visualize.
In fact, the brighter star does not have to be larger necessarily. It could very well be smaller- perhaps the larger star is a red giant, while the smaller star is a blue main sequence, which has higher luminosity.
In any case, the middle point of the
M occurs when the star with a lower surface temperature goes behind the star with a higher surface temperature, and the sides are when the opposite happens. Here's why: the amount of light that is given off per square meter of a star's surface is directly dependent on the star's surface temperature. The surface temperature is not always related to the star's size (if both stars are main sequence, then the larger star will have the higher surface temperature, but if one of the stars is a giant, that may not be the case - giant stars are relatively cool in comparison). Whenever an eclipse occurs, no matter which star is being eclipsed, the same amount of surface area is covered up (equal to the size of the smaller star). Thus, since the same amount of surface area is covered both ways, the star that has the higher surface temperature will give the deeper dips on the graph when it is eclipsed.
What this means is that the brighter star is not necessarily the one with the higher surface temperature. Here's an example: suppose you have an insanely large supergiant star that has 100,000 times the luminosity of the sun. Nonetheless, it is fairly cool - its high luminosity is due to its size. We also have a relatively small, but extremely hot type O blue star that is 50,000 times the luminosity of the sun. Now, the supergiant, even though it has a lower surface temperature, is still brighter overall. However, the same principle still applies: the smaller central dip of the
M will occur when the blue star is covering up the supergiant (in other words, when the dimmer star covers up the brighter star), and the larger dips will occur when the supergiant covers up the blue star.
See this nice eclipsing binary simulator to get a visual idea of how it works.