MKV2AC3 query on file size change?

[boredandilliterate]

Hopefully a quick one here. I have tried to find the answer to this but am a bit stuck, so thought someone here might be able to help.

I have an MKV file of approx 4.6Gb. I converted the audio from DTS to AC3 using the MKV2AC3 programme, keeping the 2 subtitle tracks, but the file size dropped to 3.9Gb in size.

I had understood that AC3 had poorer compression, therefore it should have been a bigger file? Have I mucked up somewhere, left something out or is this perfectly normal? I used 640kbps (the original DTS was 1536kbps) and Aften.

With this change, anyone think the difference in sound will be really noticeable, could the AC3 be converted as a better quality than 640, so as to be similar to the 1536kbps DTS.

I want to be able to play directly from HDD to TV, but the new TV doesn't have a DTS decoder.

Thanks for your help.

[leghorn]

AFAIK, 640 kbps is the highest possible bitrate for AC3.