4:3 DVDs - how are they resized?

[Comatose]

Well, we all know that 720x480 isn't 4:3, so how do software or even hardware players go about resizing them?

Do they completely change the resolution, change the height, or the width?

Usually players change the width, but it seems kind of a waste to downsize to 640x480...

[LoRd_MuldeR]

Don't confuse the "Resolution" of the video with it's "Display Aspect Ratio".
DVD's are either encoded at 720x576 (PAL) or 720x480 (NTSC), which obviously is neither 4:3 nor 16:9.
Nevertheless those DVD's are 4:3 or 16:9, it's called "anamorphic" encoding.

The player will read the DAR (Display Aspect Ratio) and stretch the video accordingly, so it looks correct.
For example MPlayer will "stretch" a 16:9 PAL DVD from 720x576 to 1024x576 at playback...

To do the correction only one dimension (either the width or the height) needs to be changed.
The best method (quality-wise) to display a 16:9 DVD is stretching it horizontally -- that is: increasing the witdth.
Crushing it vertically would be possible as well, but resolution would be lost.

Also keep in mind: The video needs to be scaled in fullscreen mode anyway, in order to fill the entire screen.
This scaling can be combined with the necessary DAR correction...