MacDown and MarkdownPad Pro are the editors I use on Mac and Windows too. The Pro version of MarkdownPad has nice support for GitHub flavored Markdown, either using the GitHub API or with an offline renderer.
As much as I like MarkdownPad [Pro], it has its own bloat problem. It uses Awesomium [1] for the preview HTML rendering, which can really chew up a lot of memory.
The original version of MarkdownPad used the native Windows WebBrowser control, but the author didn't know the trick that allows that control to render in IE11 mode instead of IE7 mode. I reported it on their forum over a year ago [2] but it still uses Awesomium.
Anyone who needs to use an embedded web browser on Windows should grab the code in that forum thread. Getting the native WebBrowser into IE11 mode instead of the terrible default IE7 mode makes it a useful control. Since you don't need to bundle an entire web browser with your app, it cuts down the app size quite a bit.
[1] http://www.awesomium.com/
[2] http://forums.apricitysoftware.com/t/why-is-markdownpad-spaw...