Why doesn't VKontakte compress some pictures?

Question

In contact with

It has been noticed that the VKontakte engine does not reduce some pictures, leaving the original size and volume. I made several test JPGs with different sizes (from 3000px to 900px) on the long side and with different compression ratios. I tracked the following pattern: the engine does not resample photos up to 1280 px in length, but compresses images larger than 200 kb.
There are frames with a link to the " original " on which the image is loaded 2560x1577 366kb
How does it differ from the "compressible" counterparts? Maybe a specific compression algorithm?
Who has any guesses?
PS: Here is another frame with the original size in megabytes.

Answer 1

After uploading a photo to the site, it is compressed and several versions are created (for preview, small version, large, etc.). The link to the "original" is the version with the least compression.
The maximum photo resolution at the moment is 2560x2048.
In the first example, there is no sharpness in most of the image, there are no details, which means that the compression algorithm works better.
In the second , there are more details (for example, the contours of the leaves) - the size is larger.
The third one came up in height, and was compressed at the time of processing with a frame.
There is an excellent explanation on this subject from LIVE. + at the end of the entry there are links to 2 tests.