While H.264 uses 16x16 pixel blocks to process video frames, HEVC uses Coding Tree Units (CTUs) that can take sizes up to 64x64 pixels. This allows the encoder to compress large, unchanging areas of a video frame (like a clear blue sky or a dark wall) much more efficiently.
→ Allows retaining more detail at very low bitrates than H.264. 100mb movies hevc full