It's best to think of each pixel as a little blob (or box) of color, and an image as a matrix of those pixels. When you crop, you're not
magnifying the bit of the image you want, but
removing the bits you don't.
Take a very simple example, an image that is 10x10 pixels in size, and very, very boring:
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Now, let's say we only want the middle part of this image:
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When we crop into this area, we are left with an image that is only 4x4:
View attachment 58258
If you want to keep the image at its original size, you then need to
scale or intelligently resize it, via a process known as
resampling, which will take the original pixel data and then, essentially, spread it out over a larger area, intelligently adding (
interpolating) new pixels to fill in the gaps and overlaps. You can see, below, the lack of correlation between the boxes you have, and the matrix over which you want them to sit, which is obviously not an issue if all the boxes are the same color, but can be quite a big issue of they're all different.
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Many decent editing apps will allow you to scale, and will do a pretty good job of it (although it's important to remember that they are always
messing around with the pixels that you actually captured, and adding new ones of their own, so you may lose detail, sharpness and so on).
Here's a real-world example (supersized!) of a 10x10 image being cropped and scaled—you can see how the scaling process has added in intermediary pixels with colors that are not in the original:
View attachment 58260
(And yes, technical people, I am over-simplifying a little... ;-) )