1. A method for objectively estimating user-perceived quality of an encoded video stream in dependence on the video content of the stream, comprising: generating a sequence of size values, the size values being the absolute or relative sizes of encoded video frames in the stream, the sequence being in chronological order corresponding to the video frames; analyzing the sequence of size values, prior to or without decoding payloads of the encoded video frames, to determine, based on the size values, parameters associated with content-dependent variations in user-perceived quality; and searching a reference model in order to identify a reference dataset corresponding to the parameters determined by analyzing the sequence of video sizes, the reference model comprising a plurality of reference datasets corresponding to a plurality of user-perceived quality levels; estimating the user-perceived quality of the stream based on a user-perceived quality level corresponding to the reference dataset identified within the reference model.