If you're talking about data communication bandwidth (the number of bits going over the ADSL-line or between your wireless router and the device) it's fully determined by the source. That sends the whole picture, and the browser scales it to what it needs.
With an iPad it's a little bit more complicated. With the new high-resolution display (more pixels) an app may decide to load a higher resolution picture, which has more bits to uses more bandwidth.
Do bigger screens consumer more bandwidth than smaller screens?
Or is size irrelevant and the determining factor is screen's resolution?
If screen resolution is the determining factor, would a tablet, a PC, and a laptop, each with different screen sizes but resolution set to 1024 x 768, use equal bandwidth?