Why is HDV the same bitrate as DV?

Question

How did developers find a way to squeeze 4x more resolution (HDV) into the same old miniDV tape?

Answer 1

DV uses DV25 so it's lightly compressed and almost lossless.  HDV uses MPEG-2 - it's highly compressed and  lossy.

Answer 2

The developers noticed that HDTV uses essentially the same data rate as DV. Unlike DV, which compresses each frame individually, HDTV and HDV uses Mpeg2 in which a complete frame is only sent occassionally and groups of in-between frames are interpolated. So HDV has much worse temporal resolution than DV but has a higher pixel count.