I just did a quick Google search and came across this – http://wp-snippets.com/remove-wp-caption-inline-style-width-in-wordpress-3-4-and-up/ and this – http://wp-snippets.com/remove-default-inline-style-of-wp-caption/ Give both of those a try and see if they work.
There are other posts that address this issue as well…..I googled – “remove wordpress caption width” and got the above results.
