Some people are like pictures, if you scale them up too much, they will lose their quality.
A photograph that looks sharp on a phone screen turns blocky and blurry when stretched across a billboard. The quote borrows this familiar digital fact to say that people, too, have a scale at which they do their best. Push someone far beyond the size of role, pressure or attention they suit, and their real qualities seem to thin out. It is not necessarily an insult. A small picture is not a bad picture, and the flaw lies in the enlargement, not in the image.
The author is anonymous, so there is no single thinker behind it, but it echoes old advice: the Delphic call to know yourself and the Greek ideal of nothing in excess. Wisdom, in that tradition, includes knowing your measure and respecting it, in yourself and in others.
In practice, imagine someone who is a wonderful teacher in a small classroom and is offered a high-profile administrative post because it looks like success. Before accepting, they might ask whether the new size will keep what makes them good, or whether they would be happier growing deeper rather than bigger.