Altium Default Clearance between Polygon and trace
Unless things have changed recently it's set by the clearance design rules. I have no idea what happens if there is no applicable clearance rule but it wouldn't surpise me if it chooses to use a very small clearance.

I usually set a clearance rule specifically for polygons (use "inpolygon" in the query for one of the objects) to give the clearance I want arround polygons, often quite a bit higher than what I use as my regular clearance rule.