Martin,
I think it probably depends on how large the image is when you upload it. The ‘expand’ option seems to display the image at the original size. For example my Ptolemaeus image is 1200×1755 pixels and the expanded image displays as that while on the pre-expand screen it is scaled down to fit. Consequently one appears larger than the other.
Your last image of Mars seems to be 500×550 and displays as that when expanded but does not need to be scaled down on the previous screen and therefore appears the same size in both.
Cheers,
David.