Interesting. I'm using Firefox 11 as well and can see definite differences... but if anything I would say that the enlarged image is more saturated on my screen than the smaller version (I also calibrate with a spyder)
In IE 9 both images look the same.
One thing I have noticed - using the firefox add on 'FxIF' for the smaller images reports: -
GPS Coordinate: undefined, undefined
Comment: CREATOR: gd-jpeg v1.0 (using IJG JPEG v62), quality = 80
where as for the larger image reports:-
Camera Maker: Canon
Camera Model: Canon EOS 7D
Lens: EF100mm f/2.8L Macro IS USM
Image Date: 2012-04-24 13:40:55 +0000
Focal Length: 100mm
Aperture: f/6.3
Exposure Time: 0.040 s (1/25)
ISO equiv: 100
Exposure Bias: none
Metering Mode: Matrix
Exposure: aperture priority (semi-auto)
White Balance: Auto
Flash Fired: No (enforced)
Orientation: Normal
Color Space: sRGB
GPS Coordinate: undefined, undefined
Photographer: H.Shepherd
Copyright: Copyright 2009
Software: Adobe Photoshop CS6 (13.0 20120305.m.415 2012/03/05:21:00:00) (Windows)
so clearly something is different
& maybe the forum resizing software is changing the colours
Downloading both images & viewing in photoshop, I have resampled the larger image to 1025 x 643 (the size of the smaller image) & then samples the pixel @ 200/310
From the smaller image I get RGB 169/4/34
& from the larger RGB 177/4/34
so maybe Firefox is actually rendering the differences correctly & other browsers are not!