Weird scaling-effect going on when rotating a UIView with Core Animation

Question

I'm rotating it like this:

[rotationView.layer setValue:value forKeyPath:@"transform.rotation.z"];

wrapped in a UIView animation block with beginAnimations... and commitAnimations.... Basic stuff.

That view contains a stack of three other views which are rotated also. It's a clock, basically.

What happens: When the superview of all these rotates with core animation, the inner views start to scale like a pulse. as rotation starts they get smaller (looks like they zoom out), and then when rotation ends they get bigger again (like zoomed in). that effect can make you feel drunk. well, not what I want.

What can cause this?