It is weird, especially since I used a standard preset in Compressor to convert the clips.
Here are the specs from one of the clips as copied out of the QT viewer info window:
Apple ProRes 422 (HQ), 1,920 x 1,080 (1,888 x 1,062)
Linear PCM, 16 bit big-endian signed integer, 2 channels, 48000 Hz
23.98
446.3 MB
182.88 Mbit/s