When you set that final argument of add_image_size() to false, it scales the image down until the height or width matches one of the set values, then it maintains the proportions.
For example:
- A 600px by 400px image using your settings is 134px wide (the long dimension) and then 89px tall (proportional to 134px wide).
- If the image were 400px by 600px it would be 134px tall by 89px wide.