In reference to NginxHttpImageFilterModule image_filter resize directive:

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

>>>
Content-Length:
...or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET...


When a HEAD request is done, it seems nginx returns the HEAD of the image it would of fetched before the resize, which includes the original image's size. Instead, to conform to spec, nginx could GET the image, resize, and return the Content-Length of what it would of returned.

As a work around, is there an easy way to remove the Content-Length header in the response? I guess NginxHttpHeadersMoreModule is the best way.