Quoting from Giant's website:
"Compact Road
When Giant introduced Compact Road in 1999, the impact was anything but small. Giant shortened the rear triangle and the seat tube and increased the slope of the top tube to produce a frame that's lighter, stiffer with improved handling and acceleration than conventional frames. Success leads to imitators; revered Italian frame makers and the big bike companies alike now follow Giants lead. Giant stays ahead with the new Composite Compact road."

Here's the link.
http://www.giant-bicycles.com/us/050...50.600.000.asp