Block expands more than the height of content when using collapsiblock module?
This is a wild guess (no time to test this) but you could try some of the following and see if this will make a difference:


Try using a different version of jQuery (download and install jQuery Update and then go to Configuration>Development>jQuery update and test different versions. Don't forget to clear the cache every time before the test).
Try tinkering with your CSS classes for the block and its contents to include a max-height: 100% property. For example, try this for the ID of the block (e.g. #block-block-2) and/or for the .content class inside that block (#block-block-2 .content)