I have raw file(CR2) size at 13.6 mb straight out of my Canon 7D. When I convert them in Adobe dng converter 5.7 I get 21.7 mb. When opening in Photoshop CS3 with Camera Raw 4.4.1 I get image size as W:3888 pixels by H:2592 pixels. I dont understand at which point the file size reduction takes place, as its supposed to be larger than that providing it was 13.6mb original file size. ( There is no compression option in camera raw or dng converter).
There is a hyperlink at the bottom of the Camera Raw interface that allows you to change some of the settings. One of the changes is to make the image smaller, the image that is sent to Photoshop. Is it possible that you have a setting chosen that reduces the size? Additionally, DNG files are usually a little smaller. Are you embedding the raw file in your DNG file? The DNG converter does not change the size of the image. It simply takes the raw image data and puts it in a new container DNG file.
The "size" in the converter only relates to the preview jpeg that is embedded, and shouldn't affect the size of the raw image.
I think you are converting an sRAW or mRAW. DNG currently doesn't support the format in which these are stored, so it is storing it in a more "generic" style, which is less efficient than sRAW/mRAW.