I am 21 years old and my moustache has only grown on small parts on my face. It is very thin and small. How can I grow it more?

Facial hair develops at a different rate on everyone because everyone is different.  Give it a few more years to fill in.  There is no magic formula to speed-up facial hair growth.  This is caused by genetics.  Your dad can probably shed some light on this for you. Ask him at what age he was able to grow a fuller mustache. Remember, you are made up of his genetic code.

There is nothing but time that you need on your side. There is nothing really that works to accelerate the growth of your mustache or beard as this is a genetic trait that you inherited from your parents, Ask you dad about when his beard and mustache grew and the expect the same time-frame for you. 

Some people genetically don't grow much facial hair, it may be normal for you.  At this age you may consider waiting to see if you get more hair.  If as you age you still have thinned out mustache you can consider mustache transplant.