The MacBook screenshot appears to be working as intended, and follows the conventional height from Apple’s HIG. The iPhone issue is one I hadn’t encountered. The extra space is meant to avoid the notch on newer phones, but I’ll need to see if there’s a way I can target the phones without a notch.
You can definitely make adjustments via CSS snippets. I’d recommend using the Developer Tools as seen in Getting comfortable with Obsidian CSS. It’s worthwhile to learn if you want to make customizations.