Reduced video size after muxing with mkvmerge

Hi!

I used mkvmerge to delete an audio track from an mkv file. However, the video size was also reduced from 58.2 GiB to 53.1 GiB. The audio track was 2.41 GiB. The total file size was reduced from 73.6 to 66 GiB. So clearly there is a bitrate loss.

The compression in the Extra Options tab was set to ''none''. Does anybody have any idea why is this happening? Mkvmerge should not encode the video...

Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L4.1
Format settings, CABAC : Yes
Format settings, ReFrames : 2 frames
Format settings, GOP : M=1, N=10
Codec ID : V_MPEG4/ISO/AVC
Duration : 3h 48mn
Bit rate mode : Variable
Bit rate : 36.5 Mbps -> 33.3 Mbps
Width : 1 920 pixels
Height : 1 080 pixels
Display aspect ratio : 16:9
Frame rate mode : Constant
Frame rate : 23.976 fps
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.670
Stream size : 58.2 GiB -> 53.1 GiB (80%)
Title : Video (29853 kbps)
Language : English
Default : Yes
Forced : No