It seems that your second experiment uses larger images, which have a width of 1900 pixels instead of 1000 from the first experiment.
If that’s the case and I don’t misunderstand the information, it’s expected that this use case uses more memory.
I would recommend to use the latest stable release (1.5.1) or build from master to get the latest fixes.
1.2 and 1.3 are both old by now.