Those are Adobe Flash (FLV) formatted video. From what I've seen, Youtube uses professional Sorenson-based Flash video (professional video codec), sized to 320x240 with a 425x318 playback size. They're not very good quality either. I can create On2V6-codec (Sorenson, medium-end codec) that are native 425x318, using Adobe Flash CS3 (v8) and Flash CS3 Encoder and they still are not very good viewed much larger than native size. Youtube also has some degree of mystery in their compression filesize choices and encoder settings, so video quality varies wildly.
I use this stuff where I work.