iFrame Not Downsizing

I am trying to use this site in an iFrame with iFrame-Resizer version 4.2.2:

https://dealertile.com/

However, I cannot get a consistent sizing of the iFrame. To begin with, I have to use custom 'heightCalculationMethod' or the iFrame only renders to 150px height:

• bodyOffset = 150px height.
• bodyScroll = Correct initial height. Grows on page changes, but does not shrink.
• documentElementOffset = 150px height.
• documentElementScroll = Correct initial height. Grows on page changes, but does not shrink.
• max = Correct initial height. Grows on page changes, but does not shrink.
• min = 150px height.
• grow = Correct initial height. Grows on page changes, but does not shrink.
• lowestElement = Correct initial height. Grows on page changes, but does not shrink.
• taggedElement = 0px height.

You can see my demo iFrame here: https://dealertile.com/demo/

Can someone help me figure out why the iFrame is not shrinking and only growing?

Problems like this always come down to a CSS bug, the most common issue is setting a percentage height on an element