The class .form-control-sm from bootstrap is being overwritten by theme code and not allowing small form inputs. I think it should be reasonable to expect to be able to use form-control-sm standard class from bootstrap.
I added the class to one of the form fields in your demo to illustrate the problem as shown in the screenshot attached
I have found similar issues with the .table-sm class, also attached. It seems the .table-sm class from bootstrap hasn't been included in the theme at all