My Apple TV app screen is zoomed too much on my Samsung tv

• Your TV may have a zoom function – but I presume you would have noticed that with other inputs.
• Apple TV has a ‘overscan’ zoom function (that would be up to ~4%, not that much). It would be off by default, and I haven’t seen digital 16:9 content that could warrant changing this.
• Apple TV has an accessibility zoom function to magnify for the visually impaired. Use Zoom to magnify the image on Apple TV - Apple Support. Maybe you accidentally activated this?
• Apple TV has a temporary zoom function to eliminate letterbox bars (double-tap the touch surface lightly without clicking through; double-tap again to return to normal). This is temporary to the currently playing content only; it does not stick after navigating out of it.

Do you have the Apple TV app among the TV smart apps, or do you have an external streaming device, e.g. the Apple TV box? If so, which model?

How much zoom are we talking about? Just the edge, or missing missing 50% in all directions?