x.265 small file size

[Trapper]

Anyone know why Enterprise Season 2 an original bluray rip is encoded at 1280x720 (720p) with a file size of 3.3GB.

 

Whereas the same episodes from the same BluRay are encoded with x265 are 1080p but at 350MB in size!!

 

I know the bitrate is far higher on the 720p encodes, but the quality is much better on the 10x reduced version in 265

 

Just to confuse matters Season 1 is encoded at 1080p with H.264 and that is also 3.3GB - the same size of the 720ps!

 

I am so confused.