You can scale objects proportionally in Penpot by activating the scaling tool from the preferences menu, or pressing the K keyboard shortcut. When you've enabled the scaling tool, the cursor changes from a single two-headed arrow to a double two-headed arrow. Any object you resize will be scaled proportionately when the scaling tool is enabled.
Learn UI design with our free Penpot course: https://penpot.app/courses/block-1/resizing-vs-proportional-scaling/