The front and rear overhang is the main difference size wise I can see from the pics.
The rear does look bigger to me--bigger trunk on the US-version. But the front looks identical to me.
If you study the line from the top of the flared out wheel-well running towards the headlight, the Euro Mazda6 line turns in right around the headlamp washer, whereas the N.A. version the line runs down towards the lower bumper. Giving a very different look and perception ,IMO of a more protruding nose.
From Mada press release regarding N.A. market difference-The North American MAZDA6 differs from the similarly named car sold in the rest of the world in many ways: larger interior and exterior size, more powerful engines and unique styling.
The 2009 MAZDA6 will be available only as a four-door sedan, and in three trim levels: Sport, Touring, Grand Touring, with varying levels of standard and optional equipment.
Transmission choices are a six-speed manual or five-speed Sport AT automatic with manual-shift mode (2.5-liter engine) or a six-speed Sport AT automatic with manual-shift mode (3.7-liter V6).
http://media.ford.com/mazda/article_display.cfm?article_id=28124&CFID=1053514&CFTOKEN=609a485b3e3c16e5-367FD5B1-1185-6933-5D885915254191B1