Hair seems to shrink when combing

Hi Blender Community,
I am trying to follow some blender hair tutorials and came up with a strange problem I cant get hold of. Whenever I try to comb some hair, the hair seems to shrink in size/length. Is that just a setting I have to change? The hair of the pony for example should stay 10cm in length, but I want to comb em.
Hope I could explain my problem.
Any Idea how to fix this?
Thanks in advance
Hilmar