Size isn't necessarily an indication of quality. A 150dpi JPEG converted to 300dpi will end up a larger file size but it won't be of better quality.
I know what you mean, but I'm kind of a nitpicker. DPI (Dots Per Inch)
is only really a meaningful term when talking about printing. When dealing with computer graphics in general, it is meaningless. You can save a 300x300 JPG image with, respectively, 150 and 300 DPI, and the two files will be exactly the same size, and indeed, both images will be 300x300 pixels. The DPI setting is just a small bit of metadata telling a program the default size the image should be printed as. In this case, the first image would be 2x2 inches in size, and the second would be 1x1 inch. Both images would be exactly identical, apart from the physical size.
My apologies for the rant ;-)