Image size

i created an image cache preset with a SCALE height to 300px. My problem is that it is not really used. There seems so be an internal flow scale happening.
I attached a sample image. The outer left/right pictures should have the same height. One is landscape format and one portrait format, but that should not result in different heights. The sides are smaller (not 300 in height) but the scaling should happen proportional.
The middle picture does not have a height of 300 too.

Any ideas?

Thank you