Hopefully I can give a little more context.
To explain with some images from Balsamiq.
I created a simple group of 3 boxes vertically stacked.
I then cropped them down to show just two boxes:
When I go and edit the group and add a fourth box and exit the group the crop size has changed to show 2 and bit boxes:
My expectation is that the size of the crop wouldn’t change and I’d just see 2 boxes from the 4 in the group.
Why is this important?
When showing an interface that is bounded by a mobile devices limitations at is helpful to construct the full interface and only show the visible part with a cropped group:
Any modification to the group (anything that changes the height of the group) will modify the crop size. Which in this case is not what I need to happen.