I agree with you to a certain extent. But there are many brand name 5MPcameras that use a 2/2.5 sensor. And Sony uses a 1/2.4 sensor on many of their 5MP cameras.
Here are the ones I found:
Fuji: E510, F450
Casio: EX-250, Ex-255
Kodak: DX7530, DX7590
Pentax: SV, X, S5i, S50
Sony (1/2.4): DSC-F88, DSC-T11, DSC-T1, DSC-T3, DSC-M1
I Think the sensor size is a factor in additional noise, but not the whole reason. I think the total design of the lense, sensor, and processing engine has more to do with it.
I have also noticed that some of the other big zooms (5MP) have larger sensors, but as you said... they don't maintain a f2.8 through the full range.
I also noticed that any camera that I checked that was above 5MP had a larger sesor. Maybe 5MP is the max for a 1/2.5.
I am anxious to see what the next generations of the Lumix will bring. Hopefully a larger sensor, gain up on the display, etc...
What ever the case, I have definately learned enough by now to know what I am looking for.