The EOS 650D is based on the same body design as the EOS 600D, and side-by-side they are indeed very hard to tell apart. The new model gains a stereo microphone (a first for the EOS line) and a slightly revised top-plate layout, but that's about it.
From both the front and rear, the two cameras are nearly indistinguishable. With the 650D the microphone has been moved from the front shoulder of the camera. Subtle differences in LCD screen design and shaping of the rear buttons are about the extent of external updates.
The changes that have been made are largely to the top plate. The 650D gains a stereo microphone in front of the hot shoe, and movie mode is now a third position on the main power switch. The exposure mode dial gets a couple of new options, losing A-DEP in the process, and the DISP button (which did little more than turn the screen on and off) has disappeared. Here you can also see that the new EF-S 18-135mm f/3.5-5.6 IS STM lens is a little shorter than the existing version, with a slightly different barrel design.
Compared to EOS 600D - key changes and improvements
'Hybrid CMOS' sensor with integrated phase detection AF for live view and video
Continuous AF in Live View and during video recording
Touchscreen interface including touch shutter / touch focus
9-point AF, all sensors now cross-type
High ISO range increased to 12800 standard / 25600 expanded (vs 6400 / 12800)
DIGIC 5 processor
5 fps continuous shooting (vs 3.7 fps)
Eye-sensor to toggle LCD screen on and off
Integrated stereo microphone
Compared to Sony SLT-A57
The EOS 650D is only fractionally larger than the Sony A57 and actually a bit lighter, despite the fact that the Sony lacks an optical viewfinder. Both cameras house an APS-C sized sensor and accept external flash units.
Both the EOS 650D and the Sony SLT-A57 follow a traditional SLR design and control point layout. The mode dials reside on opposite shoulders of the cameras and the 650D's shutter button sits in front of its control dial, while the A57 reverses those positions.
Both cameras feature a 3.0 inch articulated LCD, but the side-hinged (versus bottom hinged) design of the 650D makes it more useful for shooting from high angles in portrait orientation, and allows you to preview self-portraits or group shots when using a tripod. The electronic viewfinder of the Sony A57 makes it (slightly) shorter than the 650D, but does allow for a more gently-contoured shape.
Outfitted with their respective 18-55mm kit lenses, both cameras would occupy a similar amount of space in a camera bag. This view also highlights the wider and more deeply-recessed handgrip of the Sony A57.