The key phrase in Markphd statement is "In theory it is superior." This is true, in theory it is supperior. However it is all relative to the source. The maximum bit rate for DVD's with mutiple soundtracks is 448 kps acording to Dolby and the DVD liscence agreement. However if the disc is a single soundtrack it can the use up to 754kps. This is the case on some superbit disc and special dts only editions (jaws,saving private ryan, etc.)
It is not a matter of what they are capable of doing, it is what they are limited to.
As far as the DTS soundtrack being a larger file than the DD file, it is because they are differnt file types. IE different languages and differnet decoders. They file size is not the issue in compression, it's the bit rate.