vga vs HDMI

Setup: Dell 8300 with ATI Radeon HD 5670 and ST232L monitor.

Video card to monitor connection:

VGA cable connection looks very good and fill screen (23" diag).

HDMI cable connection looks not a good and does not fill screen (~21" diag).

Monitor - video card are auto detecting to the correct cable.  Resolution in both cases is 1920 x 1080 (reommended).

Any idea what I am doing wrong?