Sorry to hear about the miscarriage and this cyst. Cysts can grow at different rates. If it is truly a dermoid, it will not go away on its own and will need to be surgically removed. Here is a website that may be helpful - http://ovaryresearch.com/ovarian_cysts.htm.
I had the same thing in my right ovary. I didn't know until after I gave birth to my son, it had burst and inflamed my appendix. My gyno/surgeon told me that it likely grew very fast due to the hormones that come along with pregnancy, I was 19 at the time. You might want to see about having it removed I know for myself the aftermath of it bursting caused scared tissue and pain I still deal with not to mention I had my appendix removed also.
Best of luck!