File size of 4mp and 8mp

[Bledina]

I have a olympus 4mp and XT.The MP of XT is double of the Olympus but why the file size is not Double?For example,I took the same photo with the olympus and the XT but why the output size of the XT is not double of the Olympus.The olympus is 2.2MB and the XT is only 2.8MB(only slightly difference) :confused:
Thanks!

[Rex914]

Different manufacturers use different JPG compression techniques hence a non-linearity in file sizes.