I found the same issue here and the solution from there works: Settings in ‘Media > Settings’ is ignored when inserting images
The solution is as simple as paste this:
$content_width = 2000;// The value I want
in functions.php
Explanation copied from there(user Chip Bennett):
I believe your issue is that the value set for the global $content_width variable (which is 640px in Boilerplate and 584px in Twenty Eleven) is less than the width you’re specifying via Settings -> Media.
WordPress is overriding your user setting with the Theme-specific value. This actually makes sense, since a Theme knows its maximum content width, and using a larger image width than what the Theme is designed to accommodate would very likely break the Theme layout.