A Question About Max Pic Size vs Posted Pic Zoom Extent

This has puzzled me since I first joined our forum. How is it that some members can post pics that zoom way more than really necessary, but a whole lot...an' others' pics zoom very little, or even not at all?
Given the 1 megapixel limit (an' in my admitted ignorance of the science behind any of this stuff) I don't understand how that range of available zoom is possible. Please illuminate this dark subject for an ol' fool; an' Thanks!

 

The limit of file size for a file being attached is 1 Megabyte. It has nothing to do with megapixels (which are a different measurement altogether.)

I'm sorry to say that I am as clueless as you are about zooming.

 

Ahhh...Thanks! I had erroneously assumed that they were (roughly) equivalent.

Now, to discover how to fit lots more pixels into a megabyte. There's gotta be a way!

 

If you're uploading a .jpg or .png file it's already about as compressed as it's going to get.

 

Might have to do with pixel density? The default on my devices is 72DPI, but if someone else is at 300DPI?

If I've done this right, here's a picture at 72DPI


And the same at 300DPI

 

I take pics w my iPhone and then process them through an Image Sizer app I downloaded for free. I can make the file size just under the 1 Mb limit.

 

Seems reasonable, but both those examples zoom to exactly the same physical (screen) size...on my machine, anyway.

 

AND when BOTH are CLICKED ON to ENLARGE, they are BOTH at 100%.....