Does compressed mean, the size of the image was reduced (e.g. 1080p -> 720p) or also the conversion from png to e.g. jpg?
The answer has already been covered, but perhaps let me try a 3rd way...
Instead of pictures, imagine movies.
Lots of movies are available for download from the internet. Let's keep it simple and say we're talking about a single 1080p movie.
One download could be 16 GB, another 8.75 GB, another 4.75 GB and yet another 1.2 GB.
They're all the same movie, same soundtrack - but the data rate is different. The "compression" is different.
The 16 GB version might have an average data rate of 25 MB/s. The 1.2 GB version might be nearer to 2 MB/s.
All that means is there is more data for each image that makes up each frame of the video.
Movies (and pictures) save space by instead of storing every single pixel, they store groups of pixels that are the same color (lossless) or by storing groups of pixels that are "more or less" the same color (lossy). The newer algorithms are really clever and aren't just doing simple things like "this pixel and the pixel next to it" or even "this pixel and the 8 pixels around it" - instead they are doing complex math to reduce sections of the image down to mathematical summaries.
Ever watch a really low quality movie where there is a solid color? Maybe it's a white wall or a black night sky?
In porn games, it's usually walls or clothing. But instead of seeing what the eye would normally see as a solid single color, you instead see obvious circles within circles - the lower the quality... the more obvious those circles of graduated color. The 16 GB version of the movie would have the same gradient of color - but can use more data (more intermediate shades of colors) to paint the picture... so you don't see the edges and you don't see boundaries between each layer of color.
That's the compression we're talking about.
Most graphics conversion programs will let you tweak the quality/compression - but most people use them without altering the default values.
So it's the trade off between file size and how much of that blocky graduated color patterns you want.
For example, here's data for a 1080p picture I chose to convert at random...
.png
24 bit color - 1.9 MB
.jpg
Quality=85 - 623 KB
.jpg
Quality=60 - 359 KB
.jpg
Quality=30 - 137 KB
Same image... even the same file format in some cases... but the quality/compression means different file sizes, despite the image being the same dimensions each time.
I would usually start with
.jpg
Quality=60... then if I noticed any color pooling or other graphical glitches... I'd up things to either 85 or 95, depending on the problems the image is having.
I offer these two examples. Both 1080p. Both
.jpg
.
The first is 34 KB, the second is 13 KB.
Honestly, it was a quick and dirty example... and not really a good one, because I was using too few colors and the color boundaries are clearly visible even on the first image. But you should see what I'm talking about.