Thumbnail with different sizes

Intermediate image sizes are discrete, not variable. When you define an image size, with specific dimensions (whether hard-cropped or box-resized), WordPress will create a discrete image, with the specified dimensions.

Otherwise, if width could be defined dynamically, WordPress would have to create a prohibitively large number of images, just to account for responsiveness.

To account for responsive design, you’ll have to use CSS. The easiest way to do so is using a rule similar to the following:

#content img {
    max-width: 100%;
    height: auto;
}

That way, if the #content div is resized, your post images will be resized along with it, while maintaining their aspect ratio.