I recently noticed a colleague using a shape that had smaller radius corners than the basic rounded rectangle shape, and that the corner size remained the same as the shape was resized.

Upon further investigation, I found they were using the UML → Activity diagramming shape. And after some more digging, I saw that the UML → Interface object had even smaller radius corners.

Here is a board I put together with the different shapes I found (feel free to copy the board by clicking on the board title and using the Duplicate action):
 

While these other shape objects solve my problem of the basic rounded rectangle shape’s corners being way too big (as noted in this Wish List idea post) when I need to make the rectangle very big, the only downside I am seeing is that the font size does not scale with the shape: