Some advertisers set the width and height of the media file as the aspect ratio (ie width=16 and height=9), and if you scale the file larger keeping the aspect ratio, than it should still look good assuming that the file was saved at a bitrate that supports a larger size. According to the spec though, the width and height attributes are supposed to be the native width and height of the media file and not the aspect ratio of it. If an advertiser lists the aspect ratio instead of the native dimensions then it can cause problems because not all video players scale an image up to full screen or to dimensions that look best on the device. Instead the video player displays the video with the exact width and height dimensions provided, which in this case would be a very small 16×9 video. This leads to a poor user experience and is not what the advertiser intended. If an advertiser wants to provide different quality and size files then they should include multiple media files with accurate dimensions specified for each one.
Another variation that some advertisers deliver via VAST, is the absence of the media file width and height attributes. In this scenario advertisers may leave them out because they assume that the video player will scale the media file to look best on the device. The absence of these attributes is a violation of the IAB VAST spec though since they are required, and if the video players uses a very strict VAST parser than the VAST may not be classified as valid and will not display. Other VAST parsers may not be as rigid and will let the omission of the dimension attributes slide and will still display the video. This could result in a poor quality version of the video being shown if it was not saved properly to allow it to be scaled up and still appear correctly. So advertisers should always include the media file width and height attributes in order to ensure the video displays properly and that it meets the IAB specifications.
Although the IAB has published a VAST specification, it is not always followed by advertisers, and when they do stray from it there can be unintended consequences. The results can vary from a video that is shown with very small dimensions, to one shown with poor quality at a larger size to not displaying at all. Advertisers can be limited in the control they have over where on a page or app and on what device model their ads are shown, but by making sure their VAST conforms to the VAST specification, they can increase the probability of it displaying properly.