The diameter of the cable outer jacket only vaguely represents the electrical quality of the cable where fatter on average is better up to about 3/8 inch. Unless you ahve test equipment, you will have to go by your own ears (or eyes in the case of video cables), or go by other people's word, or go by magazine reviews.
The material that separates the center conductor from the concentric (usually braided) shield has a lot to do with the quality of the cable. Also, manufacturing bloopers, such as solder melting almost through the aforementioned material making the center conductor no longer exactly centered can influence the quality.
I would expect that with cables of modest length, about 10 feet or less, used for audio, the average person cannot tell any difference.
Cables rated for audio, namely red/white or red/white/yellow, will probably give a viewable picture even for high definition (analog) component video, but are not intended for video or digital audio. Inferior cables can result in drop outs with digital audio or digital video. If, coincidentally, video grade cable was used for all three lines in the bundle for manufacturing simplicity, then you lucked out. For A/V cables, the yellow should be better or equal; for component video cables, the green should be better or equal.
Video hints:
http://members.aol.com/ajaynejr/video.htm
