when I am writing words,such as head ,the font size will change?

[ghost]

When I am writing head,it is small,when I finished write,It become big,Is it normal? When I am writing normal text,It doesn't change.

[cloose]

Yes that is currently the behaviour. The highlighting of the markdown is asynchronous to not hinder your typing. Also the header are currently shown with a different font size.

In the upcoming release you will be able to turn off the different font sizes.