How do I tell my roommates that they are using too much soap?

I live with five other girls in a college dorm and at least two of them use too much dish soap. This may seem like a petty thing, but six girls go through dish soap fast enough without using any excess. What they do is take the soap and squirt it right onto the item to be washed, and they use as much soap on one dish as I use on my sponge to wash four or five dishes. Okay, so I admit, it is kind of petty, which is why I don’t know how to approach them. Thus, I turn to the collective.