I think the way to achieve this would be to make the content within the longer group dynamic, and keep the element heights very short. The text element heights will expand automatically, which will also make the group expand, as long as the text content is dynamic. This way you’ll be able to keep the sizing of the group itself and those text elements to be very small within the editor, and on page load, and only have the group increase the scroll when the group is visible. Similar to what Skyler mentioned, this could be done in a regular group or a repeating group. 
But the only way I can think of is to use dynamic content (creating a Thing, with fields, and a data entry) just for the longer group, even if it’s only for this purpose.
So in the editor, it’d look like this (the green is what you need, also this image is zoomed out a lot just to show the groups are the same height)
And in preview it will expand as needed:
- On page load (shorter scroll bar):
- When groups are shown (longer scroll):
Editor:
Preview: