Joomla 3 has some strange restriction on the height of a modal popup in a component view. 

My workaround is this:

In the view which uses the modal add this at the top of the tmpl.

$document = Factory::getDocument(); 
$style = ' .modal-body {height:262px;} .modal-body iframe { height:232px;}' ; 
$document->addStyleDeclaration($style);

 If the modal footer with the save/close buttons is 30px then the modal-body needs to be 30px more than the iframe. Adjust to suit the content.

If the content is wrapped in a form, as it often is, you might need to add style="margin:0;" to get rid of 18px of white space at the bottom which is the default protostar derived template.css files.