You're right, the 550D isn't exactly huge but the G3 is just that bit smaller. If you want even smaller the GX1, or the older GF2 are the next step down.
Panasonic Lumix DMC-G3 is 11% (13.6 mm) narrower and 14% (13.9 mm) shorter than Canon EOS Rebel T2i.
Panasonic Lumix DMC-G3 is 38% (28.6 mm) thinner than Canon EOS Rebel T2i.
Panasonic Lumix DMC-G3 [382 g] weights 28% (148 grams) less than Canon EOS Rebel T2i [530 g] (*inc. batteries and memory card).
Check this site:
http://camerasize.com/compare/#185,219
Pair it with the 20mm pancake lens and it's a pretty small package that should provide the same IQ as your Canon 95% of the time. You get an EVF instead of OVF but you gain a touch, swivel screen. DPreivew has a pretty exhaustive review of the camera.
I'm really happy with the lens and sensor performance, coming from entry level DSLRs, and appreciate the smaller size. I'm tempted to downsize even further now though (it's like an addiction) and probably look at selling my G3 after xmas for a GX1.
The G3 is going for good prices new now, a lot less than when it came out 6 months ago. The GF2 is even less (but uses the older 12mp sensor). Or if you're interested in a used one and in London let me know, can meet you after xmas when I'd be ready to sell and compare it to your 550D.