You might want to clarify which supplementals you're alluding to. All schools have different secondaries, some have essays and some don't, some have limits of 500 while some have limits of 5000 words etc.
So far, I haven't had too many problems with not writing enough... my problem lies with more on how I want to structure/say certain things. If you're stuck and it feels too short (usually it's all in your mind!) then you should jot down a list of things you want to mention in your essay before starting it. It usually helps me move ideas around and ensure that I talk about everything that I want to add in.
I also really believe in quality>quantity. Adcoms have limited time on your app, so I don't think that by writing dull 500 words is better than your meaningful 300.