It's all relative to your graphics settings. You have certain settings like viewing distance which regulate when details will change. It's a set distance. Turned down low enough, even mountains will disappear, no matter how many pixels wide they may be. It's especially noticeable on open world type games, you see grass looking like grass up close, then at a specific distance that field is barely recognizable as grass, and in far distances its a green/yellow solidity. A toon is much larger than a blade of grass, but the same applies, there's settings for non background items, that includes toon, and at a certain point all you see is outline, then nothing. Many refer to it as 'fog of war' etc.
A sniper scope changes nothing, only increases detail rates, but not fog distances. So if watching an enemy run away through a scope, when it reaches the detail limits, it follows suit, and when reaching fog limits will disappear totally, even if the scope makes it look huge.
Resolution won't change this, it'll only change the amount of detail possible in each range, not what those details will be.