With the right compression, 25GB of space might be all that the final encode really needs.
The newer codecs (AVC, etc) are more efficient than dvd's implementation of mpeg-2. HD is six times the resolution of SD, but only really needs 4-5 times as much space thanks to the improved compression tech. HEVC further improves on this so you only need twice as much space when you jump from HD to 4k.