Why does AVS Video converter ask to decrease video quality?

I am using AVS Video Converter and I was wondering why it asks to lower the quality to convert a .mkv file to dvd even though the .mkv file is 4.36 GB? That should be an even conversion to .iso or .vob files so why must I lower the quality? If anyone is more of a wiz with AVS Video Converter I'd be very appreciative if you could enlighten me as to why this is occurring. Thanks!