Pointed by a customer
The combo topic lists the following limitation, written a few years back - ref:
"The simple combobox does not have input for sizing its height. In the future, the IgxInputGroup component will expose an option that allows custom sizing, and then the IgxSimpleCombo will use the same functionality for proper styling and better consistency."

However, its height can be modified through the --size or --igx-input-group-size CSS variables. This limitation can be deleted.