Images resolution

I added 3 pictures in the gallery. All three were 1920x908 pixels in size. The first picture always remains in its original size and 2 others are always reduced to 1600x757 regardless of the order. There are always the images that will be reduced and others will not be reduced, even though they have the same pixel size and similar file size. What does that depend on? I read over 256 kB. The problem is that images are around 450 kB. Either they should all be reduced or they should all remain unchanged. I think that thanks to broadband internet the pictures could be twice the size. Or the user could choose the compression level himself.
Thanks,
Viktor