The composer is a modal, and as such doesn't scroll. It is full screen on Android. There is no extra space other than what's on the screen already. You can't remove the topic title bar, it's needed to create a topic, you could remove the tags, but they're a fundamental part of the system, the kitchen sink, meh. I only use it for uploading images. But again, kind of needed.
The only other option would be a two step process, so, the first modal, enter topic title and tags, click next, get a full screen text window to enter the post. Then submit.
Adding space at the bottom wouldn't make a difference unfortunately. As the modal is designed to use the full available width and height on mobile. This is not an issue on iOS, because iOS keyboards adjust the page height properly when it's loaded.