How can HEVC file be so much smaller than h264?

[Mano]

I googled and found the general answer is HEVC at most gives file 50% smaller than h264 at the same quality. However i saw people ripping blu-ray at 125mb for a 30min 720p clip while a normal h264 rip is 1GB.

[burfadel]

HEVC is more efficient than h264, just like h264 is more efficient than h262 (MPEG-2). There are so many variables to take into consideration though, including encoder and its version, the settings used, and filters used before compression etc. Also consider the audio codec used as that takes up space in the file. Most of the difference though comes down to settings, encoder used, filters etc though. Typically a 125 MB 720P file for 30 minutes h265 should look better than h264 (although it could look worse if using bad settings versus good settings for h264), but in either case the quality drop would be 'unacceptable for 720P'. In other words, if encoding at 480P, because there is less information to encode the quality could actually end up being much better as you aren't constrained so much in the allocation of bitrate.

Basically, the people encoding 720P to 125 MB for 30 minutes probably don't know what they are doing :).

[Mug Funky]

A lot of the gains come from a combination of bigger frames, having larger blocks and modern cameras being exceptionally smooth and stable.

The 50% figure is for uhd content. On average this is highly unlikely to be any source other than a gorgeous new 4k+ camera with a huge dynamic range and low noise.
When you go down to 720p, the differences are not so huge.

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