DEPENDE sa conversion ang Video Quality
for example 1080p mkv movies usually play between 8-12gb in size so anything lower than that is FAKE 1080p, oo ng 1080p reso nga pero pangit vid quality
all you need to do is use a converter that can specify the Video bitrate of the conversion from MKV to AVI or MP4, lets say the video bitrate of the original vid is 5000kps, set mo yung video conversion to the same as 5000kbps or closest, kung gusto mo 5.1 channel pa rin use a converter that can handle more than 2 Channel stereo, if sa TV lang ang speakers use 2 Channel Stereo 48,000hz Sampling, 320kbps audio para maganda quality(CD quality audio is 128kbps)
SAME video resolution does not mean same video quality UNLESS you specify that the conversion is LOSS-LESS kasi mga presets sa ibang converting software usually lower the video quality