It does appear small, and unfortunately that is an aspect of the Contacts app user interface that cannot be changed. Nor does the Contacts app allow in-app zoom (unlike Safari and many other apps).
Your only real options are to use the system-wide setting for display scaling or the accessibility functions for zooming.
System-wide setting for display scaling
- Go to System Settings > Displays
- Select an option more to the right, towards 'larger text'

Accessibility functions for zooming
- Go to System Settings > Accessibility > Zoom
- Select an option to temporarily zoom either the whole display or a picture-in-picture (loupe) option

The UI itself can only be changed by Apple. To have Apple consider your feedback, submit it here:
Feedback - macOS - Apple