Settings can be set on a syntax specific basis. It looks like you have Markdown files set to have a specific different color scheme, and in addition the draw_centered
setting is enabled for them specifically, which makes the text center in the display (thus giving you the wide margin).
The more obvious culprit for that is MarkdownEditing, which ships with default settings that override the built in Markdown.sublime-settings
to include a variety of things, not the least of which is a different color scheme and the centering of the display.
As such you could fix the problem by disabling that package or by choosing Preferences > Settings - Syntax Specific
while you have a Markdown file open and manually add in a color_scheme
setting and set draw_centered
to false to make it more like you might expect.