The slider behaves as it was designed to (since the 1.0 update) – it attempts to display the slides at their maximum height (while cropping the sides) to avoid the images becoming unreadable short slices on the narrowest of screens.
If you want to return to the previous behaviour you can use this bit of custom styling:
body .seriousslider-inner > .item img.item-image {
max-width: 100%;
}